Imperfective "хотеть(ся)" versus perfective "захотеть(ся)"

Question

With most verbs, the difference between the imperfective and perfective forms is obvious to me. But not so with хотеть(ся) and захотеть(ся), perhaps because "to want" is a state or condition, and not a clear cut action like "to read".

What is the difference between the following pairs?

1a) Мне хотелось поесть.

1b) Мне захотелось поесть.

2a) Детям хочется спать.

2b) Детям захочется спать.

3a) Ребёнок хотел конфету.

3b) Ребёнок захотел конфету.

4a) Они хотят учиться в Гарварде.

4b) Они захотят учиться в Гарварде.

Answer 1

Мне хотелось поесть.

If you say this out of context, the response would probably be когда? ("when was that?").

Imperfective verbs denote state, so хотелось means that at some time in the past, you were experiencing desire to eat something, and it's not clear whether or not you want to eat now.

Мне захотелось поесть.

This sentence makes sense by itself. Perfective verbs denote state transition, so you are saying that you have switched your inner state from "not wanting to eat" to "wanting to eat" - which means you're hungry now (or were hungry at the point in time you're telling about).

Answer 2

1a vs 1b - As Quassnoi noticed, the difference is "state transition"

3a vs 3d - same difference, as for 1a vs 1b. Word part -"ось" (and also "ся") does not changes anything here). In Russian, we call it "returning particle" or "возвратная частица". Here, it just means that subject and object of action is the same. Notice that sometimes this particle have different functions. For example "забрал" and "забрался" words have absolutely different meanings.

2a vs 2b - As noticed Matt, here "за-" changes verb tense to future.

4a vs 4b - Same as 2a vs 4b

As you can notice, preposition "за-" has two different functions here: first - changing verb tense, and second - adding "state transition". But to be honest, there are much more functions that "за-" can do with words (not only verbs). For example in these pairs of words:

делал  - ЗАделал
бросил - ЗАбросил
смотрелся (в зеркало) - ЗАсмотрелся (на кого-либо)

"за" preposition has absolutely different functions.