Infinitives are generally used with the marker to. But we use the infinitive without to in some cases. After modal auxiliary verbs After the modal auxiliary verbs will, shall, would, should, can, could, may, might and must, we use the infinitive without to.
Besides simple infinitives like (to) write, there are also progressive, perfect and passive infinitives. Progressive Infinitive Form: (to) + be + present participle Examples are: (to) be writing, (to) be reading, (to) be walking etc.
Functions of the to-infinitive The to-infinitive as the subject of the sentence. This is a formal usage and is far more common in written English than spoken:
The gerund always has the same function as a noun. A gerund can be the subject in the sentence. Examples: Walking is good for you.
Ing-forms, present participles & Gerunds (ing форми, сегашни причастия и герундий)