Verb Tense and Aspect

English verb forms are constructed from four ideas:
1. time
2. before that time
3. stronger
4. happening to the subject.

We show the “time” idea with past and present tense finite forms. These forms change for different times and for different people.
I did it yesterday; you do it most days; he does it every day.

We show the “before that time” idea with the (perfect form), which uses (have) and a [past participle]. It means that verb happens earlier than the other time.
She had gone when I arrived. I did not see her.
I have done my homework. I can show it to you.
In the second sentence, have done is before now. We do not say the time because we do not need to.

We show the “stronger” idea with the (continuous form), which uses (be) and a present participle. It means that the verb is stronger than usual. We do this when we say something is around a time (i.e. before and during and after that time), and when we talk about a length of time (e.g., five minutes, ten years), and when we talk about an action which is repeated (i.e. it happens many times), and when we want to make the verb more important.
I did not see her leave: I was sleeping. (around the time)
He was waiting for forty minutes. (length of time)
You are tapping your pen. Stop it. (repeated)
I didn’t do anything! I was just sitting there! (important)

We show the verb happening to the subject with the (passive form), which uses (be) and a past participle. It means that the verb does not go from the subject to the object: the verb goes back to the subject. We can use “by” to say who does it, but we do not need to say that.
The fish was eaten by the cat.
The window is made of glass.