When to use or not use "a" after a verb? E.g. "voy a comer" and "quiero comer"

Question

In English we think of the infinitive as being "to" + action. E.g. "to eat".

There are cases where we use a verb followed directly by the infinitive.

E.g. "quiero comer"

quiero = I want
comer = to eat
quiero comer = I want to eat

However there are also many cases where we put "a" before the second verb.

E.g. "voy a comer" or "empiezo a comer"

empiezo = I start
a = to
comer = to eat
empiezo a comer = I start (to) to eat

Are there rules about when to include the "a" when to leave it out?

Answer 1

"A" precedes the following verb most of all in "perífrasis verbales". A lot of "perífrasis verbales" require a preposition (a, de...) or some other conjunction (like que, in tengo que). Some examples are:

• Empezar/comenzar/iniciar/principiar a [verbo]: empiezo a comer
• Ir a [verbo]: voy a comer
• Ponerse a [verbo]: me pongo a comer
• Volver a [verbo]: vuelvo a comer
• Llegar a [verbo]: llego a comer

However, some don't. Some "perífrasis verbales" that don't require an "a" are:

• Deber [verbo]: debo comer
• Soler [verbo]: suelo comer
• Poder [verbo]: puedo comer

There are also certain "perífrasis verbales" that take the gerund form of the verb. Almost none of them require the "a":

• Estar [verbo]: estoy comiendo
• Seguir [verbo]: sigo comiendo
• Continuar [verbo]: continúo comiendo
• Llevar [verbo]: llevo comiendo [una hora]

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"A" doesn't precede the following verb most of all when the verb can be replaced by a pronoun (i.e., when it's a subject or a direct object):

• Quiero comer — "comer" is a DO, and so it can be replaced by "eso": quiero eso
• Me gusta comer — "comer" is the subject, and so it can be replaced by "eso": me gusta eso
• Merezco comer — "comer" is a DO, and so it can be replaced by "eso": merezco eso
• Conviene comer — "comer" is the subject, so it can be reaplaced by "eso": conviene eso

Look how the replacement by "eso" doesn't work with the "perífrasis verbales" cited above:

• Empiezo a eso (wrong).
• Voy a eso (wrong).
• Me pongo a eso (wrong).
• Vuelvo a eso (wrong).
• Llego a eso (wrong).
• Debo eso (wrong).
• Suelo eso (wrong).
• Puedo eso (wrong).
• Estoy eso (wrong).
• Sigo eso (wrong).
• Continúo eso (wrong).
• Llevo eso (wrong).

Although some instances with "eso/ello" instead of the verb in "perífrasis verbales" are idiomatic (such as "ponte a ello", "a eso vengo" or "ni a eso llego"), they are at least obscure from a grammatical point of view.

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Bottom line: if you can susbstitute the verb by "eso", don't use "a". If you can't, use "a". If you do this, you will be right 3 out of 4 of the times. Three exceptions are "soler", "deber" and "poder", which don't require "a" even though you can't substitute the verb by "eso". Never use "a" before a gerund.