The imperative is often perceived as a tense, when in reality it is a mood. It is used

to express command e.g.
entre, entrez! come in!

Or suggestion e.g.
Bâtissons! let's build!

To use it simply take the je, vous or nous forms of the verb on their own.
The nous form is used as let's…. In English. The je and vous indicate be!

Some verbs however use the present subjunctive for this.
The most common ones are avoir and être.

They are shown below

To form the let him…..., let them.…. In French, take the present subjunctive form, with Que, qu' in front of it.

Qu'ils mangent du gâteau! Let them eat cake!