If a verb comes directly after por or para, then yes it is generally in the infinitive (Necesitas practicar para aprender.... You need to practice in order to learn.... Por no dormir suficientemente, te vas a enfermar.... Because of not sleeping enough, you are going to get sick). But the difference between when to use por and when to use para is much deeper. Some tricks would be when numbers follow, you generally use por. When you are dealing with substitution or something to do with "because' in English, you use por. If you are dealing with "in order" in English like (I will eat in order to live), then you use para. It you are dealing with a "gift" you use para. An example would be... She needed to go to the store and buy a book. But she got sick. So I went to the store and bought the book FOR her. (por, substitution)..... She loves reading. So I went to the store a bought a book FOR her (para, gift).
