I have always known that after a preposition you have to use the following verb in the gerund form, however, it seems I have wrong al this time and I would like to know the reason. Sentence: "I have to leave for work now", as far as I know, this sentence is well constructed, but why?
Objects of prepositions (like any other object) have to be nouns or noun phrases.
Many (but not all) words in English can function as verbs or nouns, and the meanings will typically be related. Work is one of those. Because of this, you don't need to change it by adding -ing to it.
The usual way English causes a word to only be a verb is by putting a subject in front of it - I walk to the park - I makes walk unquestionably a verb.
The usual way English causes a word to only be a noun is by putting a determiner, like a possessive pronoun or article in front of it - I walk to the park - the makes park unquestionably a noun.
This does not mean articles are strictly required in prepositional phrases and there's numerous well-known phrases that don't take articles, like in bed.