What concord principle do we usually use for the existential sentence?

Why is past tense often used for politeness?

What concord principle do we usually use for the existential sentence?

Using extraposition:

Whether they come or not doesn't matter very much.

Using inversion:

I have never been there before.

Using an appropriate cohesive device:

The professor stepped into the classroom. The professor opened his book and then began to talk.

Using passive voice:

We imported from abroad a lot of machinery.

Using an adjunct:

Set your alarm clock, or you will oversleep.

Using passive voice:

Mary seems to like John very much.

Using a relative clause:

They will go to large cities. They think they can find good jobs there.

Using a non-finite form:

I knew that he was poor. I offered to pay his fare.