Translation:We have not seen each other since Wednesday.
I am sure there is a faaaaar better explanation to this, but the first thing that pops into my head is that:
Nós = we
Nos = us (+ourselves/each other)
Let me know if you find an exception, but I think this is the simple way of explaining the difference.
(edit: thanks, paulenrique, I added your suggestions to the explanation!)
One's the subject, one's an object pronoun. That's the general idea, but there's a bit more to it.
"nós" is used as the subject of a sentence and after prepositions (para nós; to us, for us), whereas "nos" is used as the object of an action. "Eles nos usam para aprender portugues"; "They use us to learn Portuguese"
Do take what I say with a grain of salt. I'm still learning and it's perfectly possible that something I said is completely wrong.
Well, the thing is , in portuguese we do not have the "do" and "did" to change the verbal time of the sentence, so , somethings get a little hard to understand and explain, and sometimes it is cofusing.
I had problems to get it at first when i started to learn english.
But in this case for example, if you check both sentences in google translator or somewhere else, both will have the same translation.
It's not a stupid question at all. I can't give you the rule, since I'm learning this from the ground up like everyone else around here, but I've seen these enough to recognize a certain pattern. "Desde" is the trick to this sentence. It carries the temporal weight. Just like in English, if you use the word "since" then you are referencing to some past event or idea. Once you have that in place, the tense on the verb is much less important. In English we happen to use the present tense "have", but the meaning exists in Portuguese without it. The literal translation "We not each other see since Wednesday" is the proper grammar in Portuguese. Through tradition we say in English, "We have not seen..." In both cases, the word desde/since is the important reference.
(Note: Do not confuse this use of "since" with a common Americanism. Most of us often use "since" to mean "because," but that isn't correct (in formal English). Ex: "Since you know how to do this, I won't explain," would not translate to "Desde você..." but rather "Porque você..." EDIT: Or "Já que voce..." is okay, too. Go figure!)
Well, the davidalso made a real good anwser. But i will try to explain a little more.
On our language (portuguese) we use have literaly, like:
She has a dog. I have a pen . etc.
It is not normal in portuguese use have if you are not saying some kind of possessive sentences. It is not wrong, but it is just uncommon.
And if you use it in a sentence that will change the verb , in english you normaly only changes the time of the verb to the past -ed and some exceptions, to a going action -ing and some exceptions and is it, i know you have will and other stuff to determine another times, but the verb itself still the same, in portuguese we have many more variations.
In this case if you want to use "have" the sentence would be : Nós não temos nos visto desde quarta-feira.
So the verb "see" (ver) , becomes from "vemos" to "visto" in case you add "have"(ter) to the sentence.
In english that will change the sentence to " We have not seen each other since wednesday"