I read that after the phrase "used to" we use gerund. I read this sentence: "He does not remember how Mike used to worry him."
When I read this sentence, it gives better sense to me (with worry and not worrying) But I am an English student so I want to make sure is this sentence correct? If yes, then why gerund rule (that I mentioned above)has not been used?
A gerund after "used to"?
There's a different construction that uses a different sense of the verb:
He is used to working long hours.
This is very much like "he is accustomed to working long hours."
Mike used to worry him
This is very much like "Mike has worried him, but doesn't worry him now.
Note that "is used to" uses grammar typical of the passive voice -- some form of the verb to be and a so-called past participle. The preposition "to" followed by a gerund makes sense in that case.
We can even find examples like this:
Those newspapers are used to wrap fish.
Even with the passive voice form, the sense of the verb used here is more like employed than accustomed.
Not every use of "used" is the same.
The "to" in "used to" is the "to" that starts an infinitive. That's the whole purpose of the "to" in "used to," to be the first half of an infinitive (i.e., "to" + verb; e.g., to be, to go, to run, etc.), so what always follows "used" in the context of "used to" is always an infinitive. The only reason we split the infinitive in the definition of "used to" by tacking the "to" onto the end is to make it clear that it's not referring to any of the other uses of the word "used."
A gerund, nor a present participle, (i.e., a word ending in -ing) never follows "used to."
You might be confusing A) "used to" + <base form> with B) "be used to" + <gerund>.
A) I used to wake up early. (I woke up early before, but I don't now)
B) I am used to waking up early. (I'm accustomed to waking up early)
These are two different grammar structures with different functions. The example you provided is structure A). The rule you mentioned is structure B)
