Not getting this at all

Is there a particular preposition that gives you trouble or is the pronoun? 

For a + pronoun, I think it's more about 1) emphasis since it's redundant and optional, or 2) for clarification when there are 2 subjects in a sentence.

E.g. Los profesores me han dado un certificado is the same as Los profesores me han dado un certificado a mí. 

E.g. ¿A ti te afecta la crisis? is the same as te afecta la crisis but with emphasis: for you personally, has the crisis affected you?

"Le hemos explicado a ella el problema" uses 2 subjects "we" and "her".  However, if you didn't have any context for this sentence we wouldn't know if "le" refers to a "him" or "her".  So a pronoun is used to clarify that we are explaining the problem to her (a ella).

For por/para + pronoun, i think it's super helpful to understand the uses of por vs para before tackling this.  If you know when to use them, it will make the pronoun part SO much easier.  

I recommend this lesson on spanishdict tbh.  It's so clear
https://www.spanishdict.com/guide/por-vs-para

I think the rest of the prepositions often follow the english sentence structure and should be easier to figure. But I'm happy to try to explain more if you have specific questions