How do I form a positive imperative command in French?

The French imperative is used to give commands and is formed differently depending on whether you are using the tu, nous or vous form.

To form a positive command in the nous or vous forms, just take the present tense form of that verb and delete the subject (ie. the nous or the vous).

For example, this means that the vous imperative form of ouvrir would be "ouvrez", and the nous imperative form of the same verb would be "ouvrons". 

Note that the nous form of the imperative takes a suggestive meaning: "let's open" for example.

The formation of a tu command is slightly different. As for the vous and nous forms, you take the present tense form of the verb and delete the subject (tu). However, if this form happens to end in es or as, then the final s must be deleted.

For example, if we take the verb aller: the tu form of the present tense is vas, which ends in "as" and so this final s must be deleted. This means that the imperative tu form for aller is "va".

On the other hand, for the verb finir, the tu form of the present tense is finis, which does not end in either "as" or "es" and therefore the s is retained. This means that the imperative tu form for finir is "finis".

Watch out, there are four verbs with irregular imperative forms: être, avoir, savoir and vouloir.

être

tu: sois   vous: soyez   nous: soyons

avoir

tu: aie   vous: ayez   nous: ayons

savoir

tu:   sache   vous: sachez   nous: sachons

vouloir

tu: veuille   vous: veuillez 

Note: the imperative of nous for the verb vouloir is not used.