First, you are going to need to know the difference between conjugated verbs and infinitives. You can review here.
In Spanish sometimes it is hard to choose if you need an infinitive or a conjugation. Here are some rules to help you out.
Las Reglas:
1. All sentences must have a conjugated verb based on the subject of the sentence.
Yo bailo.
Tú siempre cantas.
2. If there are two verbs in a simple sentence, one immediately following another, the second one can be “left alone” or, in other words, remain as an infinitive.
Yo quiero bailar.
Te gusta cantar
3. However, compound sentences can have multiple conjugated verbs. Compound sentences occur when a conjunction (and, or but/y, o, pero) or a comma is present.
Yo bailo y canto todos los días.
Carlos practica deportes los lunes pero descansa los martes.
Pablo toca el piano y Ana escucha la música.
* Note that the subject can change as in the previous example.
4. If we combine the rules we can end up with more complex sentences
Yo quiero trabajar pero ellos nunca trabajan.
Grammar Websites
SpanishDict.com- A description of some uses of infinitives after verbs.
Spanish.About.com- This website contains descriptions of grammar and usually has good examples.
IPFW- University of Indiana/Perdue University website that has detailed grammar explanations on a low tech site. Great for advanced students and students that like the full story.
Practice Activities
Practice quiz- From SpanishDict.com. Pretty advanced.
Practice activity- From IPFW. Again very advanced.