Use the Subjunctive in all direct commands for usted, ustedes, and nosotros and in negative commands for tú and vosotros.

Tú
Camina más despacio. - Walk slower. - Indicative
No camines tan rápido. - Don't walk so fast. - Subjunctive

Usted, Ustedes
Díga(n)me mentiras. - Tell me lies. - Subjunctive
No me diga(n) la verdad. - Don't tell me the truth. - Subjunctive

Vosotros
Comed con nostros. - Eat with us. - Indicative
No comáis los frijoles. - Don't eat the beans. - Subjunctive

Nosotros
Comamos. - Let's eat. - Subjunctive
No comamos. - Let's not eat. - Subjunctive

Note: The verb ir is an exception to the rule for the nosotros form!
Vamos al parque. - Let's go to the park. - Indicative
No vayamos al cine. - Let's not go the movies. - Subjunctive

Indirect commands are used to state a wish or hope that something will happen or that someone will do something. Indirect commands are commonly used when referring to the third person (singular or plural). They are usually translated by "let," "may," or "have."

An "indirect command" is formed using the formula below:

que + subjunctive = indirect command

Que pase.
Let him come in. Have him come in.

Que me llamen.
Have them call me.

If the subject is stated, usually in order to emphasize or clarify the subject, it follows the verb.

Que pase ella.
Let her come in.

Que me llame Ana.
Have Ana call me.

Que no vea mamá.
I hope Mom doesn't see.

This construction can also be in the second person, with an implication of encouragement or hoping

Que le vaya bien.
I hope you go well.

Que tengas éxitos.
I hope you succeed.

Que todo les salga bien.
I hope everything goes well for you all.

Indirect Commands vs Imperative

There is no third person imperative, so indirect commands are the only option.

Que vengan cuando puedan.
I wish they come when they can.

For the second person, the imperative gives a direct order, while indirect commands offer encouragement, guidance, or the speaker's own hope/wish sometimes using words like: ojalá, deseo (I hope)

Ojalá traigas a tu hermana.
I hope you bring your sister.

Or almost as a direct order: quiero, necesito (I want/need). The indirect command is softer, less emphatic than the imperative.

Necesito que traigas a tu hermana.
I need you to bring your sister.