With an appropriate modal verb:

(1)Children under 12 years of age in that country ________ be under adult supervision when in a public library.

(2)You can't imagine that a well-behaved gentleman ________ be so rude to lady.

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.