Where does 'Vámonos' come from?
Aunque yo uso el palabra 'vámonos' y lo entiendo, no sé porque se dice así. ¿En qué tiempo es?
Gracias!
3 Answers
Vámonos is a bit of an exception as far as I recall.
The nosotros commands (let's....) are normally formed with the 1st person, plural, present tense of the subjunctive mood.
hablemos (let's talk) [the title of one of our Categories here on SD.]
So one way to say let's go is vayamos. (1st person, plural of Ir in the present tense of the subjunctive mood).
However, another way to say (let's....) is vamos a +verb infinitive (similar to the near future periphrasis for nosotros)
Vamos a bailar. (let's dance)
Vamos a ir, (meaning let's go), for whatever reason, isn't used much. Simple vamos (present tense, indicative) is used for let's go. Of course, if it is used pronominally with the reflexive pronoun nos the "s" is dropped as with other verbs that form the nosotros command resulting in: vamos+nos=vámonos.
addendum:
I probably should have called to your attention that even though these commands are tanslated as let's .... notice that they follow the rules for 2nd person (you) commands in that pronouns are attached to positive commands. (which is why it is vamos+nos). Also like with 2nd person commands the pronouns are not attached in negative commands. So it would be No nos vamos in the negative (not no vámonos).
You would think that looking at the conjugation chart, that "Let's go" should be either "vayamos" or "vayámonos". But for whatever reason, it is much more common to hear "vamos" and "vámonos". It's probably just one of those things where common usage trumps the standard grammar rules.
I think it comes from vamosnos with the 's' dropped 
