How do you form the gerund in Spanish?

Take the infinitive, drop the ending and add -ando to -ar verbs (hablar becomes hablando) or -iendo to -er and -ir verbs (comer becomes comiendo, vivir becomes viviendo). When the stem ends in a vowel, -iendo becomes -yendo so the i does not sit awkwardly between two vowels: leer becomes leyendo, oír becomes oyendo. A handful of -ir verbs change their stem vowel: dormir becomes durmiendo, pedir becomes pidiendo.

Can you use the Spanish present progressive for the future?

No, and this is the classic English-speaker error. English happily says 'I am flying to Madrid tomorrow', but Spanish does not. Estoy volando a Madrid mañana is wrong. For a planned future Spanish uses the plain present (vuelo a Madrid mañana) or ir a plus the infinitive (voy a volar a Madrid mañana). The progressive is only for what is happening right now.

What is the difference between estar comiendo and como?

Estoy comiendo zooms in on the action as it happens: right now, this minute, food is going into my mouth. Como is the plain present and covers habit, general truth and the near future: 'I eat', 'I do eat', 'I am eating (these days)'. Spanish leans on the plain present far more than English does, so when in doubt the simple form is usually the safer choice.

The Present Progressive

The Spanish present progressive is estar in the present tense plus the gerundio (the verb form ending in -ando or -iendo). It maps, roughly, onto the English "is/are -ing".

Estoy comiendo. (I am eating.)
María está hablando por teléfono. (María is talking on the phone.)
Los niños están jugando en el parque. (The children are playing in the park.)

That "roughly" is doing a lot of work. The construction looks identical to the English one and is used in far fewer places. Get the form right first, then learn where Spanish actually wants it.

Forming the gerund

The regular pattern is simple. Drop the infinitive ending and add the gerund ending:

Verb type	Drop	Add	Example
-ar	-ar	-ando	hablar → hablando
-er	-er	-iendo	comer → comiendo
-ir	-ir	-iendo	vivir → viviendo

So -ar verbs take -ando, and both -er and -ir verbs take -iendo. That covers the large majority of verbs.

The -yendo spelling

When the stem of an -er or -ir verb ends in a vowel, plain -iendo would leave an i stranded between two vowels (le-iendo), which Spanish does not allow. The i turns into a y:

leer → leyendo (reading)
oír → oyendo (hearing)
traer → trayendo (bringing)
construir → construyendo (building)

The rule is purely about spelling and pronunciation, not about meaning. If you can hear that i would clash with the vowel before it, write y.

Stem-changing -ir verbs

A group of -ir verbs that change their stem vowel in the present tense also change it in the gerund. The e becomes i and the o becomes u:

pedir → pidiendo (asking for)
servir → sirviendo (serving)
decir → diciendo (saying)
dormir → durmiendo (sleeping)
morir → muriendo (dying)
venir → viniendo (coming)

Only -ir verbs do this. The equivalent -ar and -er stem-changers (pensar, poder) keep a regular gerund: pensando, pudiendo is rare but regular in stem.

When Spanish actually uses the progressive

Here is the part English speakers skip. Spanish uses estar + gerundio for an action that is genuinely in progress at the moment of speaking - or across a clearly bounded current stretch of time.

No puedo hablar, estoy conduciendo. (I can't talk, I'm driving.) - happening this second.
Este mes estoy estudiando mucho. (This month I'm studying a lot.) - a bounded current stretch.

For everything else - habits, general truths, scheduled events - Spanish prefers the plain present:

Trabajo en un banco. (I work / I am working in a bank.) - a habitual fact, so plain present, not estoy trabajando.
¿Qué haces los fines de semana? (What do you do at weekends?) - habit, plain present.

The English "-ing" form is the default for the present moment; the Spanish gerund is the marked, emphatic choice. When you are not specifically underlining "right now, as we speak", use the simple present.

The big mistake: the progressive for the future

This is the error to burn out of your Spanish. English uses "-ing" for scheduled future events all the time:

I'm flying to Madrid tomorrow.
We're seeing them on Friday.

Translate those word for word and you get wrong Spanish. Estoy volando a Madrid mañana is simply not how it is said. Spanish has two correct options for the planned future:

Plain present: Vuelo a Madrid mañana. (I fly / am flying to Madrid tomorrow.)
ir a + infinitive: Voy a volar a Madrid mañana. (I'm going to fly to Madrid tomorrow.)

Rule of thumb: if the action has not started yet, the progressive is wrong. The gerund describes only what is unfolding now.

Beyond estar: seguir, llevar and ir + gerund

The gerund pairs with other verbs to add shades of meaning. These are common and worth knowing.

seguir + gerund - "to keep on / still be doing":

Sigue lloviendo. (It's still raining / it keeps raining.)
Seguimos esperando. (We're still waiting.)

llevar + time + gerund - "to have been doing for (a length of time)":

Llevo dos horas estudiando. (I've been studying for two hours.)
Llevamos un año viviendo aquí. (We've been living here for a year.)

This one is gold for English speakers, because it replaces the clumsy "have been -ing for". Note that Spanish uses llevar in the present where English uses a perfect.

ir + gerund - "to be gradually / progressively doing":

Voy entendiendo el problema. (I'm gradually getting to grips with the problem.)
Las cosas van mejorando. (Things are slowly getting better.)

A quick reference

Construction	Meaning	Example
estar + gerund	action in progress now	Está comiendo.
seguir + gerund	still / keep on doing	Sigue lloviendo.
llevar + time + gerund	have been doing for (a time)	Llevo dos horas estudiando.
ir + gerund	gradually doing	Voy entendiendo.

Common mistakes English speakers make

Using the progressive for the future. Covered above and worth repeating: no estoy volando mañana. Use the plain present or ir a + infinitive.

Using the progressive for habits. "I'm studying Spanish" meaning "I study Spanish (as a habit)" is estudio español, not estoy estudiando, unless you mean right this minute.

Forgetting the -yendo spelling. Leiendo and oiendo are wrong. The vowel clash forces leyendo, oyendo.

Regularising the stem-changers. Dormiendo and pidiendo feel right by analogy but are wrong: durmiendo, pidiendo. Only the -ir stem-changers do this, and only in the gerund and a few other forms.