you use "to" to express an infinitive: to go, to swim, to walk, etc. when you have a phrase like "I have to go" , then have is the auxiliary and it's already a verb which depends on the subject: I have...he/she/it has..after this construction you have to use the infinitive "to" but there are many cases in which you have to use "TO": - using a indirect question with "whether/if" or after verbs of thought, speech,etc.(to ask, to consider, to teach, etc) : He didn't know what TO do. (he didn't know what he should do). - you use to if you want to shorten a subordinate clause which uses "so that", "such that, ""in order that": fresh air helps the flowers TO grow (fresh air helps the flowers so that they grow) - after the verbs "to seem" and "to appear" : She SEEMED TO be at home. - after ordinal numbers, superlatives like: the first, the only: Mary was THE FIRST to arrive. - after too + adjective': She is TOO young TO go to the party. - after for +object: It's easy for MARY TO do that. I hope I could help you a bit! *greetings*