Imperatives - Informal and Formal
The affirmative informal (tú) commands use the present indicative Ud. form:
Cuenta tus beneficios.
Count your blessings.
Habla más lentamente.
Speak more slowly.
The negative informal (tú) commands use the present subjunctive (yo form + if -ar, use -es; if -er -ir, use -as).
No cuentes tus beneficios.
Don't count your blessings.
No hables más lentamente.
Don't speak more slowly.
Irregular informal commands
The following eight verbs have irregular familiar commands in the affirmative:
decir - di
salir - sal
hacer - haz
ser - sé
ir - ve
tener - ten
poner - pon
venir - ven
Commands with direct and indirect objects:
With all affirmative commands, the object pronouns are attached directly to the end of the imperative form of the verb.
Compre Ud. el anillo. Buy the ring. Cómprelo Ud. Buy it.
If both direct and indirect object pronouns are attached, the indirect object pronoun comes before the direct object pronoun.
Cómprelo Ud. Buy it. Cómpremelo Ud. Buy it for me.
Note that in order to maintain the original stress of the verb, a written accent is often needed. If the command has more than one syllable, a written accent is required when one or more pronoun is added.
Command: compre (two syllables)
If the command has only one syllable, a written accent is only necessary when two pronouns are added.
Command: haz (one syllable)
hazlo (one pronoun, no accent required)
házmelo (two pronouns, accent is required)
Negative commands with direct and indirect objects
With all negative commands, the object pronouns come before the imperative form of the verb.
No compre Ud. el anillo. Don't buy the ring. No lo compre. Don't buy it.
If there are both direct and indirect object pronouns, the indirect object pronoun comes before the direct object pronoun.
No me lo compre. Don't buy it for me.
Formal commands (yo form with 'opposite' endings: -ar, use -e; -er -ir, use -a)
The formal commands (both affirmative and negative) use the present subjunctive verb form:
no hablen Uds.
no coma Ud.
no digan Uds.
As with the present subjunctive, the following verbs are irregular:
(dar) dé, den
(estar) esté, estén
(ir) vaya, vayan
(ser) sea, sean
(saber) sepa, sepan