The term imperative, from latin imperare, to command, as well as the term command form are missleading, because the imperative is rarely used to give an order. A much better term would be prompt form or something like that. The table below shows some contexts where the imperative is used.
Someone is prompted to do something that he would do anyway.
Do, what you can't stop doing.
Someone is encouraged.
Go ahead, dare you!
If one is part of the group, it is even possible to command something to oneself
You better don't do that.
In order to command something to someone it is necessary to communicate with this person directly, verbally or in writing. With a third person it is impossible to communicate directly and therefore an imperative in 3rd person singular doesn't exist.
An own imperative exists only for the 2nd person singular in Portuguese and its formation is very easy. It is formed by leaving out the final -s of the infinitive. Howecer since in brasilian Portuguese there is no second person singular, no imperative has its own form in brasilian Portuguese. All the forms of the imperative, the affirmative commands as well as the negative commands, see below, are formed with the conjunctive. The only exception is the second person singular, that doesn't exist in Brasilian Portuguese.
tu falas => fala
you speak => speak
tu partes => parte
you go away => go away
tu ficas => fica
you stay => stay
tu pagas => paga
you pay => pay
The irregular forms must be learned by heart. It is a little bit unpleasant, but the irregular verbs are always the most important verbs as well.
tu fazes => faz
du machst => mach
tu queres => quer
du willst => will
tu és => sê
du bist => sei
tu tens => tem
du hast => hab'
tu trazes => traz
du bringst => bring
tu vens => vem
du kommst => komme
The imperative, the negative as well as the affirmative one, is formed with the conjuntivo in Brasilian Portuguese.
Você and the verb in 3rd person singular correspond in Portuguese to the second person singular. A distinction between a formal context, unknown persons, and informal context, friends, is not made. To keep it short and simple, in Brasilian Portuguese it is like in English. Everybody is treated with you / você, see 5.
It would have been possible to use você with the second person singular, that still exists in Portuguese in opposite to the second person plural, but this option was eliminated in the course of history, although some people in the south of Brasil do that. Today the use of the second person singular together with você doesn't comply with the standard grammar. The disappearance of the second person plural of the verb is actually a strange phenomenon. This happens as well in some parts of South America.
(Acually a second person plural imperativ existed in Portuguese, but got lost a long time ago even in European Portuguese and is only mentioned in scientific grammars. Nowadays it is not used any more and therefore not mentioned in learning grammars.)
The following table shows the system in European Portuguese. For Brasilian Portuguese the second person singular, tu, is irrelevant.
verbs ending in -ar: falar
European Portuguese and some regions of Brasil where the second person singular is still used.
In Brasil, except the regions where the second person singular is still used
European and Brasilian Portuguese
European and Brasilian Portuguese, since the second person plural of the verb disappeared and the form vós falai, that would be a genuine imperative, is not used any more.
verbs ending in -er: vender
let's sell it
verbs ending in -er: partir
The negative imperative or negative command it formed with the conjunctive. We illustrate that with irregular verbs. For the formation of the conjunctive with regular verbs see 12.2.1.