The use of modals is not easy to master. It is true that they are not a large group, but they are very important and they have their own rules -that is why we need to become familiar with them as soon as possible.
Here is a short video with a basic introduction to modal verbs.
In the next few days we are going to study each one of them separately, beginning today with "can" and "could". Can and could are only used in the present and past; for the rest of the tenses, including the infinitive, we use "to be able to". For further exercises with modals try this excellent Modal Verb Tutorial.
Have a look at this video, I am sure it will help you understand.
