Здравствуйте

I was wondering recently what the differences between the aspects are when used in the infinitive. I mean, whenever I use an inifinitive in Russian, I seem to either use the "wrong" one, or if I choose the right one, I won't be able to explain why it's right. There are probably differences in meaning, but what would they generally be?

Eg:

Я хочу работать/поработать.
Он хочет писать/написать письмо

Sorry if these examples don't suit the context of the question.
Спасибо большое за вашу помощь
Джек