Verbs Accompanied by Prepositions
Hello again - I'm back with my 3rd question of the week! 
I highly doubt that this question has a simple answer, but here it is: why must some verbs (aprender, enseñar, empezar, acercarse, invitar, aspirar, etc.) be accompanied by the preposition "a" before infinitives, while many other verbs do not require this "a"? For example... "aprender a escribir en español" vs. "decidir escribir en español". Is that just "the way it is" or is there some grammatical explanation for the presence or absence of "a" before infinitives?
Another quick question: is there any reason why some verbs use "en" to mean "of" like with "consistir en" to mean "to consisit of"? I'm sorry if these questions are too general or stupid... These topics have been sparking my curiousity lately.
4 Answers
Hello again - I'm back with my 3rd question of the week!
Unfortunately, there is not easy trick here. But think of this anyway:
Take a verb like "to like", and make a sentence with a noun as an object: "I like this". Now replace the noun with a gerund, which is used like a noun: "I like dancing Salsa". Everything makes sense, right? Ok, let's do it with "to want" now. We first make a sentence with a noun, like "I want this". Now let's use a gerund: "I want speaking Spanish". Same thing, right?
Now, coming back to the orignal question... The verbs you are asking about are different. Let's classify them according to their syntactic behaviour:
1) acercarse
2) aspirar
3) enseñar, invitar, decidir
4) aprender
The verb first group is pronominal, and pronominal verbs are intransitive, so they usually have a complement with a preposition. No surprises here (but the preposition has to be learnt). The verb in the second group, even though it is not pronominal, simply requires the preposition for that particular meaning, like in English, where "aspire" forms a phrasal verb "aspire to + infinitive" (you don't "aspire things"). In both cases, the preposition is is used not matter what follows: a noun, a subordinate clause with "que", an infinitive,... If you get a preposition wiht a simple noun, you will also get it with more complex structures, such as infinitives and subordinate clauses.
The third group are simply transitive verbs, but some of them (eg. enseñar, invitar) can take an special optional complement with "a" to indicate what is a human direct object going to receive from you, eg. "enseñar a Pepe (a nadar)", "invitar a Pepe (a un helado/a comer)". The preposition is "a" is used not matter what follows: a noun, a subordinate clause with "que", an infinitive,... "Decidir", on the other hand, has no use for this extra prepositional complement, because you don't decide people "on things" or "to do things", so you just use it normally: "Decidí una cosa" (using a noun: "una cosa"), "Decidí invitarte" (using an infinitive), "Decidi que no quería" (using a clause with "que").
The fourth one is the most peculiar and tricky, because it is normally transitive, except when you use infinitives (then, you have to use "a" too): "aprender algo" (using a noun), "aprender que la vida es dura" (using a subordinate with "que"), but "[del]aprender vivir[/del]" is wrong, it must be "aprender a vivir". Verbs like this are not very common, but because of their similarities with the English "learn to live", English natives think that this "a" is the same as "to", and they wonder why this "a" is not used with every verb. Well... "aprender" is rather an exception in Spanish, rather than the norm.
In any case, if you want some sort of justification for this, the direct object of "aprender" is the knowledge that you acquire or memorize through experience or study, but "nadar" is not a piece of knowledge, but the act of moving through water. The preposition "a" is used to indicate that you are not simply acquiring some piece of knowledge, but trying to learn how to perform an action. If you say "aprendí nadar", someone will answer "Oh, I already knew that word" (ie. nadar). It is all about knowledge, and the knowledge learnt is the word "nadar". If you say "aprendí a nadar", it is all about learning how to perform an action.
Another quick question: is there any reason why some verbs use "en" to mean "of" like with "consistir en" to mean "to consisit of"? I'm sorry if these questions are too general or stupid... These topics have been sparking my curiousity lately.
You won't believe how many Spanish natives have asked at some point in their lives: Why do you say "consist of" and not "consist in", like in Spanish? (I am one of them). By the way, you can also say "consist in", but the meaning is different.
In the sentence "consist of", that "of" doesn't mean "of", as such. It is a completely unpredictable preposition, like "en "in Spanish, that goes along with the verb and it has to be memorized in either language. For starters, If you say
The team consists of three people
You could have also said:
The team comprises three people
So why not "The team consists three people", without a preposition? ("The team is comprised of" is a passive construction, but you can't say "The team is consisted of" anyway)
The verb "consist" comes from Latin, and it meant originally "to stand firm". It is no wonder that we say "consiste en", then. In Spanish, it means that "the team stands/rests on three people", which is why we use "en" (Eng: "on"). In English, the original meaning and conception of the verb has changed along with the preposition.
Anyway, enough ranting and digressing for today.
Okay, well, the new update seemed to wipe out all of the responses after Janices, so I am going to rewrite a question that I submitted earlier in this thread today that got erased. Here it is: I recently read on this site's dictionary that "resistir a algo" means "to resist something". I don't understand why that "a" is neccesary... I thought "resistir" was a transitive verb, so "resistir algo" would be fine...?
When I see your questions in the forum, Nick, they are the first ones I open
Keep asking!
Hey, perhaps I should be collecting the answers, too, to save myself a $53.00 grammar book purchase
I'm sorry if these questions are too general or stupid... These topics have been sparking my curiousity lately.
Hi Nick
Judging by Lazarus´s answer, the question doesn´t look too stupid now, does it, hehe. 
