hatwash answered why the two green squares above and below the right-most 2 were safe. Here's why the top-left and bottom-left squares were safe:
a b c d
2 3 4 2
0 e f g
Because of the left-most 2, only two of a,b,e are mines.
That means only one of c,f is a mine - the third mine of the 3.
That means only three of b,d,e,g are mines - the remaining three mines of the 4.
But both d and g cannot be mines, because combined with the one mine in c,f that would put three mines around the right 2. So only one of d,g is a mine.
So if only one of c,f is a mine, and only one of d,g is a mine, that means b,e are both mines - the remaining two mines of the 4. So b,e can be flagged as mines.
That means the left 2 is complete, so a can be opened safely. Also the mine at e completes the bottom-most 1, and you can proceed from there.