He used to build model airplanes.When used to is preceded by a be (am, is, are, was, were, been) verb, used to can be followed by either a simple form of the verb or the -ing form. However, there is a big difference between the meanings of sentences with the simple form and with the -ing form.
Stephanie used to study until 1:oo a.m.
Cement is used to make the walls stronger.
I am used to seeing her every day.
In the first example, the sentence is in the passive voice. The subject of this sentence is the object used by someone one or something to accomplish or do something. In other words, in the first sentence, the cement is used to accomplish the goal of making the walls stronger. We do not know who or what is using the cement. This sentence is in the present tense in the passive voice. If we did know who or what, it would follow the preposition by and be placed after the verb.
Cement is used by engineers to make the walls stronger.In the second example, the sentence is an active voice sentence. The subject (the one doing the acting) is at the beginning of the sentence. The sentence expresses the idea of something that was habitual and true in the past, so it expresses past tense. This sentence uses the -ing form of the verb.