Imperative is used in commands.

Singular imperative

You can easily form imperatives: just drop the last n from singular form and you have the imperative.

opiskelen -> opiskele!
I study -> study!
muistan -> muista!
I remember -> remember!

Negative imperative, how to forbid

Negative imperative is used when denying someone. The main verb is without any ending:

Älä tupakoi!
Don't smoke
Älä juokse!
Don't run!
Älä yritä!
Don't try!
Älä tule lähemmäksi!
Don't come closer!
Älä ota sitä!
Don't take it!

In plural it's älkää. Plural form is used for formal situations.