It depends on the sentence..There isn't really a way to know how and when to use them. Although, gerund's sound more natural, using an infinitive can *sometimes* make you sound... "hoity".. They mean the same thing. Here are some examples: Gerund- Reading is my favorite hobby Infinitive- To read is my favorite hobby The gerund sounds 100% more natural Gerund- I like eating Infinitive- I like to eat Both of these sound completely natural! Some times it's only acceptable to use one or the other. Example: I want to eat -infinitive. I want to eating- gerund. This is 100% incorrect. do not say this. If you're using "want" then you'll use an infinitive! This also goes for "need" as another example. Check out these two links, they list verbs that require a gerund and verbs that require infinitives :) http://www.englishpage.com/gerunds/gerund_list.htm http://www.englishpage.com/gerunds/infinitive_list.htm

This is not an easy or a short topic, unfortunately. Could you write a Notebook with some sentences you're having trouble with? It will be easier to see the kind of trouble you have, and for people to tell you how to understand it.