About the three units of the subject of English speaks this resumn

Unit 1: I used 

To indicate something that happened or happened in the past in a habitual way we use the modal verb "used to"

Also, it is used for something that was once true but is no longer true.

For something that was previously true but is no longer used

"Be used to" means "used to" When used as an adjective for things that are   familiar, normal or common 

In any verb tense you can use

Unit 2: Gerund vs infinitive 

We always need to use the infinitive or the gerund, when a verb follows another verb

The gerund or the infinitive as an object to use we can, subject or complement a phrase, but in general,

The gerund as a subject to use is much more common

 Unit 3:  The Passive Voice

We use the passive voice when we want to give more importance to the action and not to whoever¹ made it.

With the auxiliary verb "to be" and the past participle of the verb the passive voice is formed

When we do not know who has done the action we use the passive voice