why is the subjunctive used rather than the imperative?

Question

Regarding the examples below, why is the subjunctive used rather than the imperative?

No cierres la ventana por favor, tengo calor.
Don't close the window please, I am hot.

Señores, cierren las carpetas ahora.
Gentlemen, close your folders now.

Thank you, James

Accepted Answer

Hola James,
When we give a negative command to someone (that someone being an "informal" you, a "formal" you, to ourselves, ... the form that that command takes is the present subjunctive. This is the case of the first sentence:
No cierres la ventana por favor, tengo calor.Don't close the window please, I am hot.
When the command is affirmative and directed to a "formal you" singular or plural (usted, ustedes), that command also takes the form of the present subjunctive. That is the case in the second sentence:
Señores, cierren las carpetas ahora.Gentlemen, close your folders now.
Here are the specific lessons dealing with different commands. Have a look:
Affirmative and negative commands for usted/ustedes/nosotros/nosotras
Negative commands for tú/vosotros/vosotras
(Note that it is necessary to also revise the lessons on how to form the present subjunctive.)
Affirmative and negative commands for usted/ustedes/nosotros/nosotras
Negative commands for tú/vosotros/vosotras
If you want to go deeper into our lessons under "the imperative", you can also have a look here and if you scroll down you will see all lessons related to it. 
El Imperativo
I hope this helps.
Saludos
Inma

Answer

Hola Inmatú explicación es muy claro. Entendiendo la diferencia entre los modos imperativo y subjuntivo es uno de los sujetos los más difíciles para mí. Gracias, Jaime