¿Cuál es la diferencia entre com y em ?Puedes dar oraciones como ejemplo.

Well, it's something like
com≈with
em≈in/at

But don't take it as an absolute rule, because it isn't!

We use it in Portuguese to form sentences in a different way of English.

I mean, reading your sentence "what I want you to do", I couldn't think any translation that uses "em" or "com" (the best translation I'm thinking is "O que eu quero que você faça") , because we use different elements to form the sentences.

It might look difficult, but I think that the best way to learn it is like a child:

Just identify those elements in sentences and how they transmite meaning, and remember they.

Hope it helps 🙂