Why does Spanish put a preposition after verbs like pensar and soñar?

Because those verbs simply govern a fixed preposition, the same way English verbs do - we say 'think about', 'rely on', 'dream of' without asking why. Spanish just chooses different prepositions from English, so the mapping feels arbitrary: pensar en (think about, literally 'think in'), soñar con (dream about, literally 'dream with'), depender de (depend on, literally 'depend of'). The preposition is part of the verb's dictionary entry and carries no separate meaning you can reason out. The only workable approach is to learn the pair as a unit - pensar-en, soñar-con, depender-de - rather than translating the English preposition and hoping.

How do I know which preposition a Spanish verb takes?

You memorise it with the verb, because there is no dependable rule. That said, a few soft patterns help. Verbs of starting, going towards and learning tend to take a: empezar a, aprender a, acostumbrarse a. Reflexive verbs of emotion and separation often take de: alegrarse de, quejarse de, olvidarse de. Verbs of mental focus lean towards en: pensar en, insistir en, fijarse en. But these are tendencies, not laws - preocuparse takes por, soñar takes con, and no pattern predicts those. Treat the preposition as part of the verb from the first time you meet it, and drill the pair together.

Which Spanish verbs take no preposition where English uses one?

Five very common ones catch English speakers out because English adds a preposition and Spanish does not: buscar (to look FOR - busco mi llave, not 'busco por'), pedir (to ask FOR - pedí la cuenta), esperar (to wait FOR - te espero), mirar (to look AT - mira la luna) and escuchar (to listen TO - escucho la radio). In each case the English preposition is baked into the Spanish verb, so adding por, a or a translation of 'for' is an error. Say busco trabajo, not 'busco por trabajo'; escucho música, not 'escucho a la música'.

Verbs with Prepositions

Some Spanish verbs will not stand next to their object without a specific small word wedged in between. You do not acordar a name, you acordarse de a name. You do not pensar a problem, you pensar en a problem. You do not depender your parents, you depender de your parents. The preposition is fixed, it is compulsory, and - this is the part that hurts - it almost never matches the one English would use.

This is the regimen preposicional: the preposition a verb governs. English has exactly the same phenomenon (we rely on, dream of, look for, think about, and no one can say why), so the concept is not foreign. What is foreign is the specific pairing. There is no rule that turns the English preposition into the Spanish one. You think about something; Spanish thinks in it. You dream of something; Spanish dreams with it. You depend on something; Spanish depends of it.

The only method that works is to stop treating the preposition as a separate word to be translated, and start treating it as part of the verb - pensar-en, soñar-con, acordarse-de - learned and drilled as one unit, exactly the way you learn that mano is feminine. This page groups the highest-frequency offenders by preposition, then flags the traps and the verbs that fool you by taking no preposition at all.

Verbs that take DE

De is the workhorse preposition after verbs, especially reflexive verbs of memory, emotion and separation.

acordarse de (to remember) - ¿Te acuerdas de aquel verano? (Do you remember that summer?)
olvidarse de (to forget) - Me olvidé de las llaves. (I forgot the keys.)
darse cuenta de (to realise) - No me di cuenta de la hora. (I didn't realise the time.)
depender de (to depend on) - Todo depende de ti. (It all depends on you.)
tratarse de (to be about / a question of) - Se trata de dinero. (It's about money.)
dejar de (to stop doing) - Dejé de fumar. (I stopped smoking.)
alegrarse de (to be glad about) - Me alegro de verte. (I'm glad to see you.)
quejarse de (to complain about) - Se queja de todo. (He complains about everything.)
enamorarse de (to fall in love with) - Se enamoró de ella. (He fell in love with her.)

Note enamorarse de: English falls in love with, Spanish falls in love of. And dejar de + infinitive is the standard way to say "stop doing something" - deja de gritar (stop shouting).

Verbs that take EN

En clusters around verbs of mental focus, insistence and turning-into.

pensar en (to think about) - Pienso en ti todos los días. (I think about you every day.)
insistir en (to insist on) - Insistió en pagar. (He insisted on paying.)
fijarse en (to notice / pay attention to) - Fíjate en los detalles. (Notice the details.)
tardar en (to take time to) - Tardé una hora en llegar. (It took me an hour to arrive.)
consistir en (to consist of) - El examen consiste en tres partes. (The exam consists of three parts.)
convertirse en (to become / turn into) - Se convirtió en un problema. (It became a problem.)
confiar en (to trust) - Confío en mi hermana. (I trust my sister.)

Two to watch: pensar en means think about, not "think in" - and consistir en means consist of, where English uses "of" and Spanish uses "in". Confiar en is trust with no preposition in English at all.

Verbs that take CON

Con follows a small but very high-frequency set, several of them about relationships and reliance.

contar con (to count on / rely on) - Cuenta conmigo. (Count on me.)
soñar con (to dream about / of) - Sueño con viajar. (I dream of travelling.)
casarse con (to marry) - Se casó con un francés. (She married a Frenchman.)
cumplir con (to fulfil / meet an obligation) - Cumplió con su deber. (He did his duty.)
encontrarse con (to run into / meet) - Me encontré con Ana en el metro. (I ran into Ana on the metro.)

The two traps here are famous. Soñar con is dream with where English dreams of, and casarse con is marry with where English marries someone flat, no preposition. Se casó con María is literally "he married with María" - never drop the con.

Verbs that take A

A is the preposition of starting, moving towards, learning and daring. Most notably, any verb of beginning or learning takes a before a following infinitive.

aprender a (to learn to) - Aprendí a nadar de niño. (I learned to swim as a child.)
empezar a (to start to) - Empezó a llover. (It started to rain.)
atreverse a (to dare to) - No me atrevo a decírselo. (I don't dare tell him.)
negarse a (to refuse to) - Se negó a firmar. (He refused to sign.)
acostumbrarse a (to get used to) - Me acostumbré a madrugar. (I got used to getting up early.)
ayudar a (to help to) - Me ayudó a mover el sofá. (He helped me move the sofa.)
volver a (to do again) - Volví a leer el libro. (I read the book again.)
asistir a (to attend) - Asistimos a la reunión. (We attended the meeting.)

Two carry hidden meaning. Volver a + infinitive does not mean "return to" here - it means to do something again: volví a llamar is "I called again", not "I returned to call". And asistir a means to attend, where English attends something with no preposition at all: asistí a clase (I attended class).

Verbs that take POR

Por governs a compact group centred on worry, struggle and choosing.

preocuparse por (to worry about) - No te preocupes por mí. (Don't worry about me.)
luchar por (to fight for) - Lucharon por la libertad. (They fought for freedom.)
optar por (to opt for / choose) - Optamos por quedarnos. (We chose to stay.)
preguntar por (to ask about / after) - Preguntó por ti. (He asked about you / after you.)

Mind preguntar por, which means to ask after someone or about them - not to ask for an object. To ask for an object is a different verb entirely (pedir, and it takes no preposition - see below).

The classic traps: different preposition, or none in English

These are the pairings that generate the most mistakes, because the English speaker reaches for the English preposition and gets it wrong.

pensar en = think about (not "en" = "in"). Pienso en el futuro.
soñar con = dream of / about (not "with"). Sueño con un mundo mejor.
depender de = depend on (not "of"). Depende de la situación.
casarse con = to marry (English uses no preposition). Se casó con él.
asistir a = to attend (English uses no preposition). Asistió a la boda.

There is no logic to extract here. Pienso en, sueño con, dependo de - the pairings are simply memorised, and the sooner you stop trying to reason from the English, the fewer of these you will get wrong.

The reverse trap: Spanish verbs that take NO preposition

The mirror-image error is just as common. A handful of everyday Spanish verbs already contain the English preposition inside their meaning, so adding one is wrong. English says "look for", "wait for", "listen to"; Spanish bakes the preposition into the verb and takes a direct object.

buscar = to look for - Busco trabajo. (I'm looking for work.) Not "busco por".
pedir = to ask for - Pedí la cuenta. (I asked for the bill.) Not "pedí por".
esperar = to wait for - Te espero. (I'm waiting for you.) Not "espero por ti".
mirar = to look at - Mira la luna. (Look at the moon.) Not "mira a la luna" for objects.
escuchar = to listen to - Escucho música. (I'm listening to music.) Not "escucho a la música".

The temptation is to translate the English preposition and slot in a por or a. Resist it: busco mis llaves, not "busco por mis llaves"; espero el autobús, not "espero por el autobús". (The one wrinkle is the personal a, which appears before a specific person object - espero a mi madre, miro a los niños - but that is the personal a doing its job, not a preposition governed by the verb. See the cross-link below.)

Quick reference

Verb	Preposition	English	Example
acordarse de	de	remember	Me acuerdo de ti.
olvidarse de	de	forget	Se olvidó de todo.
depender de	de	depend on	Depende de ti.
dejar de	de	stop doing	Dejé de fumar.
enamorarse de	de	fall in love w.	Se enamoró de ella.
pensar en	en	think about	Pienso en ti.
insistir en	en	insist on	Insistió en pagar.
tardar en	en	take time to	Tardé en llegar.
confiar en	en	trust	Confío en ti.
contar con	con	rely on	Cuenta conmigo.
soñar con	con	dream of	Sueño con viajar.
casarse con	con	marry	Se casó con él.
aprender a	a	learn to	Aprendí a nadar.
empezar a	a	start to	Empezó a llover.
volver a	a	do again	Volví a leerlo.
asistir a	a	attend	Asistí a clase.
preocuparse por	por	worry about	No te preocupes por mí.
luchar por	por	fight for	Luchan por la paz.
buscar	(none)	look for	Busco trabajo.
pedir	(none)	ask for	Pedí la cuenta.
esperar	(none)	wait for	Te espero.
escuchar	(none)	listen to	Escucho música.

Common mistakes English speakers make

Translating the English preposition. "I think about you" tempts pienso sobre ti or a literal pienso a ti; it is pienso en ti. "I dream of a house" tempts sueño de; it is sueño con. The English preposition is a trap, not a clue.

Adding a preposition to buscar, pedir, esperar, escuchar and mirar. These take a direct object. Busco por trabajo and espero por el bus are calques from English "look for" and "wait for". Say busco trabajo, espero el bus.

Dropping the con after casarse and soñar. English marries someone flat and dreams of something, so learners drop or swap the con: "se casó María", "sueño de viajar". Both need con: se casó con María, sueño con viajar.

Forgetting a before an infinitive after empezar, aprender and volver. Verbs of beginning and learning govern a before a following infinitive: empezó a llover, aprendí a conducir, volví a intentarlo - never "empezó llover" or "volví intentarlo".

Confusing preguntar por with pedir. Preguntar por is to ask after someone; pedir is to ask for a thing, with no preposition. Preguntó por ti (he asked after you) is not pidió por ti. To ask for the bill is pidió la cuenta.