El Imperativo

In English, the imperative is simply formed by the omission of the subject pronoun (you). In Spanish, however, recall that omission of pronouns is commonplace in all tenses, and there are formal and informal you's.
  Usted Vosotros Ustedes
Affirmative present (él/ella/usted) subjunctive drop r add d subjunctive
Negative no + subjunctive
This table probably looks very confusing, but it will make more sense if we show you an example. (Note that each of the alterations listed in the table are performed upon a verb's infinitives):
  Usted Vosotros Ustedes
Affirmative habla hable hablad hablen
Negative no hables no hable no habléis no hablen
Note how all the informal affirmative forms are irregular, but the entire negative imperative is regular? Remember: There is NO Spanish verb that is irregular in the negative imperative! Now let's look at ER and IR verbs:
  Usted Vosotros Ustedes
Affirmative come coma comed coman
Negative no comas no coma no comáis no coman
  Usted Vosotros Ustedes
Affirmative vive viva vivid vivan
Negative no vivas no viva no viváis no vivan
It perhaps looks strange due to the fact that it is the only tense that is conjugated differently between affirmative and negative, and also note that there is no first person imperative (though technically there is a first person plural imperative - see Subjunctive), because one cannot command oneself to do something (unless you address yourself as "you", which is second person in any case!). However, as with all tenses, there are a few irregularities which are found solely in the second person informal affirmative imperative ():
Verb 2nd Person Singular Familiar Affirmative Imperative
poner pon
exponer expón
disponer dispón
tener ten
mantener mantén
detener detén
obtener obtén
venir ven
hacer haz
decir di
salir sal
ir ve
Often, however, commands are used with object pronouns, such as "Give me" or "Tell me" (note how often "me" is used); the same applies to Spanish, but in the affirmative, such pronouns are attached to the imperative conjugation just as they would to an infinitive, whereas in the negative they follow standard "placement protocol". Let's take the example commands above:

Dame - Give me
Dime - Tell me

Note how these verbs - because the imperatives are one syllable ("da" - "dar"; "di" - "decir") - do not require accents. However, if we had said "Give me it" or "Tell me it", then there would be an accent:

Dámelo - Give me it or Give it to me
Dímelo - Tell me (it)

Note: in English, the "it" in the second sentence is most often omitted, but in Spanish is used to refer back to a specific thing. Consider the following:

I want to know how you feel about this. Tell me.
In Spanish, this would be:
Quiero saber lo que sientes de esto. Dímelo.

The "-lo" refers back to "lo que", the "what". Also note that the rule about using a reflexive pronoun when two objective pronouns beginning with l are added to the end of a word (such as comprárlelas becoming comprárselas) also applies to the affirmative imperative. Thus, "Give them to her" would be dáselas. Also note that when using an objective pronoun with an affirmative imperative, you must put the objective pronoun on the end of the imperative; however, when using the negative imperative, place it as normal, such as in the following example:

¡No me mires así! - Don't look at me like that!

When using the first and second person plural affirmative imperatives (see Subjunctive for the former), drop the ending s or d respectively when adding a pronoun to the end (the only exception being the vosotros affirmative imperative of irse, which retains the d: idos):
¡Quedémonos aquí! - Let's stay here!

Also note that though the second person plural affirmative imperative replaces the r on a verb infinitive with a d and this is the technical way it should be spoken, colloquially, the infinitive itself is most often used: For instance, instead of using the technically correct form of comed for comer, speakers and informal writers will almost always use comer itself instead; note however that in this case, one still must drop the last r when adding a pronoun (except for irse: iros): Quédaos - Stay.