Why is past tense often used for politeness?

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.