With an assertive, non-assertive or negative word:

(1)If you're ________ in Shanghai, do give me ring.

(2)Surely ________ knows where the documents are.

subordination

participles

concord

How is aspect related to time?

the infinitive

Reinforcing the negation:

There's no excuse for such behavior.

Using a cohesive device:

Keep the big apples in the basket and eat the small apples first.

Why is the past tense often used for politeness?

Using subjunctive mood:

She lost her passport and got stuck at the customs.

Using an exclamation:

It is great fun to have a picnic.