[QUOTE=isadov]
is it correct to say that Am scale is the same C scale just starting in a different note (with the formula shifted 2 steps) ?
The Ami scale contains the same notes as the Cmaj scale because Ami is the relative minor of Cmaj.....

why are we looking at them that way and not just as C scale ?
Even though they contain the same notes: the Cmaj scale is a major scale with a major 3rd and the Aminor scale is a minor scale because of it's flatted 3rd.

what scale is a certain chord progression like C - Am - F - G ?
Because C-Ami-F-G are 4 chords from the key of Cmaj....the Cmaj scale would work well over these chords.

when will i refer to that progression as C or as Am ? or when it will have a sad (minor) sound and when it will have a happy (major) one ?
That would depend on the scale you used

I hope this helps you itay.

itay[/QUOTE

here's a lesson on the chords that accompany the C major scale:
guitartricks.com/lesson.php?input=11389