When to use "to" in a sentence?

When to use "to" in a sentence?
 

In Spanish, " listen to the conversation" is translated as "escuche la conversacion". Why isn't it "escuche a la conversacion?" When do you use "a" in Spanish, and when do you not?

The preposition 'a' is used a lot, and it doesn't always mean 'to'.

Answering just the question about why it doesn't appear in the Spanish sentence you first presented is simple to explain.

All verbs have patterns that I suggest you memorize. That way you'll always know whether or not a preposition is needed, and which one works.

Here is the pattern for the meaning used in the first sentence you wrote.

escuchar algo = listen to something
Notice that the preposition we have to use in English doesn't appear in the Spanish pattern for this particular verb. Memorize the pattern.
Conjugate the verb appropriately, add a direct object and you're good to go.

In contrast:

escuchar a alguien = listen to somebody.

It seems more logical in Spanish than it does in English.

Is "a" only used in this case because a person was involved?

Yes. :thumbsup:

So what's the reason that "turn to the left" is "doble a la izquierda" rather than "doble izquierda"? There's no person involved so why do we use "a"?

This time you're asking about an intransitive verb (it has no direct object). In the case you cited, the verb 'doblar' is followed by a prepositional phrase.

Verbs are classified into usage patterns. When you learn a verb, learn it with some context that'll help you to use it correctly. In other words, if a verb takes an object, learn it with an example of that type of object.

escuchar algo
escuchar a alguien
escribir/decir/dar algo a alguien
doblar a la izquierda/derecha
nevar

As Rusty said, there are two kind of verbs: transitives (eg "escuchar") and intransitives (eg "salir"). The first ones don't need a preposition, while the second ones do.

On the other hand, we have the "personal a"; a preposition used exclusively when referring to human or humanized living beings.

In the case of the verb "doblar", that "a" is indicating direction: "to" in the sense of toward ("hacia").

A pleasure.