What is the personal a in Spanish?

When the direct object of a verb is a specific person (or a personified animal or institution), Spanish inserts a in front of it. Veo a María (I see Maria) - the a is grammatically required, not optional. Compare with veo la casa (I see the house) - no a, because the object is a thing. The personal a is invisible in English translation and one of the easiest details to forget. It also appears with the indefinite pronouns alguien and nadie when they function as direct objects: no conozco a nadie aquí.

What is the difference between por and para?

Por is for cause, means, exchange and movement through; para is for destination, purpose, deadline and recipient. Lo hice por ti (I did it because of you, on your behalf) vs lo hice para ti (I did it to give to you, for your benefit). Por la mañana (in the morning, during the morning) vs para mañana (for tomorrow, by tomorrow). The split takes time to internalise; the headline is destination/purpose = para, reason/means = por.

Spanish Prepositions

Six prepositions handle 90% of the work in beginner Spanish. Each has a primary use, a few secondary ones, and a small set of phrasal combinations worth memorising.

de

The most common preposition in Spanish. Primary uses:

Possession / origin: el libro de María (Maria's book), soy de Inglaterra (I'm from England)
Material: una mesa de madera (a wooden table)
Topic: hablamos de política (we talked about politics)
Time of day: a las tres de la tarde (at three in the afternoon)

a

Direction, destination, time, and the personal a.

Direction / destination: voy a Madrid (I'm going to Madrid)
Time point: a las cinco (at five o'clock)
Indirect object marker: le di el libro a María (I gave the book to Maria)
Personal a: veo a María (I see Maria) - when the direct object is a person, a is mandatory

en

In, on, at - covers all three English prepositions of location and time.

Location inside / on: en casa (at home), en la mesa (on the table), en Madrid (in Madrid)
Time within: en enero (in January), en 2020 (in 2020)
Means of transport: en coche (by car), en tren (by train), en avión (by plane). Exception: a pie (on foot), a caballo (on horseback).

con

Accompaniment and instrument - "with".

With (someone): voy con mi hermano (I'm going with my brother)
Instrument: come con un tenedor (he eats with a fork)
Special forms: con + mí → conmigo, con + ti → contigo. Vienes conmigo (you're coming with me).

por and para

The two prepositions English collapses into "for". Spanish splits the work cleanly.

Por = cause, means, exchange, movement through:

Cause / reason: gracias por la ayuda (thanks for the help)
Movement through: caminamos por el parque (we walked through the park)
Means of communication: te llamo por teléfono (I'll call you by phone)
Exchange / price: lo compré por diez euros (I bought it for ten euros)
Duration / approximate time: por la mañana (in the morning), por dos horas (for two hours)

Para = destination, purpose, deadline, recipient:

Recipient: este regalo es para ti (this gift is for you)
Purpose: estudio para aprender (I study in order to learn)
Destination: salgo para Madrid (I'm leaving for Madrid)
Deadline: para el lunes (by Monday)
In someone's opinion: para mí, está mal (in my opinion, it's wrong)

The classic test case: trabajo por ti (I work because of you / on your behalf) vs trabajo para ti (I work for you - you're my employer).

The two mandatory contractions

Two combinations force a contraction with the masculine singular article el.

a + el = al: voy al cine, not "voy a el cine"
de + el = del: el coche del profesor, not "el coche de el profesor"

These aren't optional. They only apply to the article el, never to the accented pronoun él: lo compré para él stays uncontracted.

Verb + preposition combinations

Some Spanish verbs require a specific preposition, and the choice doesn't always match English.

Spanish verb	Preposition	English
ir a	a	to go to
empezar a + inf	a	to start to
aprender a + inf	a	to learn to
pensar en	en	to think about
soñar con	con	to dream of/about
depender de	de	to depend on
acordarse de	de	to remember
casarse con	con	to marry

The headline mismatches: pensar en (not "pensar sobre"), soñar con (not "soñar de"), depender de (not "depender en").

Worked examples

Voy al cine con mis amigos. (I'm going to the cinema with my friends.)
El libro es de María. (The book is Maria's.)
Vivo en una casa pequeña. (I live in a small house.)
Este regalo es para mi madre. (This gift is for my mother.)
Gracias por todo. (Thanks for everything.)
Veo a mi hermano todos los días. (I see my brother every day.)

Common mistakes English speakers make

Forgetting the personal a: veo María instead of veo a María. Using en for time-points when it should be a: en las tres instead of a las tres. Defaulting to para for everything that translates as "for" - half the time it's por. And missing the al / del contractions: a el and de el are always wrong with the article.