Инфинитив без частицы to употребляется:
после модальных глаголов. My dog can sit and stay.
после глаголов let, make, see, hear и feel. I made him copy out the sentence one hundred times. НО: употребляем инфинитив с частицей to после be made, be heard, be seen etc (пассивная форма). He was made to copy the sentence one hundred times.
Note: когда за see, hear и watch следует форма –ing, в пассивном залоге ничего не меняется. They saw me talking to the police. I was seen talking to the police.
после had better и would rather. We had better call ahead and reserve a table.
глагол help может употребляться с инфинитивом с частицей to или без нее. Doug helped me (to) learn my lines.