An infinitive can be a to-infinitive or a bare infinitive (without to). There is no difference in meaning between them; some structures require a to-infinitive, while others call for a bare infinitive:
I ought to call them. (to-infinitive) I had better call them. (bare infinitive)
Modals include modal verbs, semi-modal verbs (also called marginal modals) and other modal expressions. They combine with main verbs and modify their meanings. A modal may have several different meanings, while similar meanings may be expressed by using different modals:
