7: *If*
was a good move, or even an okay move, and white ignored it, then
at q17 or r16 would be a great move. (In general it's true that if you make a threatening move and white ignores it, it's great to add a second move in the general area. But if it was a lousy threat to begin with, that complicates the calculations.) ------------ Also, in general when players are taking big points in an empty part of an empty board, regardless of what their strategy is, the point is on the 3rd or 4th line. (Why? Well, play more games and you'll get an intuitive feel for it. But the short answer is that if you loosely box out a bunch of territory on the 5th line, it's very easy for white to jump under and live in "your" territory.)
15: Adding another move around c7 is considered quite big, because the extra space on the side makes it nearly impossible to kill your group.
33: Here, I don't think this was urgent (especially because locally, you end in gote! Yuck.) The life of your stones wasn't at stake, since they are connected to the top side, and white can't improve the health of his stones much, either. Instead, R10 is the last huge point on the board, impossible to attack very effectively, and threatens to hem in Q15.
37: Tiny. Again, white doesn't have to make eyes in the corner... he can escape out to the left side. Any empty part of the board would be better.
39: Consistent with 37, but also small. If you're going to try to make good on this threat, start with B13, removing W's escape route. If he ignores that, then see if you can kill.
47: Not too bad, but this would be a good time to take R12, a crucial point to make R9 safe. (Tactically, W can still play K16 and then M16, or vice versa, so you have what we call "bad aji" - there is a cloud of doom hanging over this spot.)
51: You want to play closer to D11.
53&55: This may seem like a victory, but note that you had to play two moves which only barely rescue D11 (and do little else), whereas white got to play two moves that were good locally *and* solidified his control of the center.
59: The board is so open! Why would you play here?
65: This move is just far enough away to be useless.
83: If you want to keep some (potentially living) stones from getting boxed in, play L7.
87: As emeralddemon pointed out, this is just unnecessary.
Okay, past this point I think there are few open areas left, so our theme breaks down.
At this point, I don't necessarily think your biggest problem is keeping groups/strings connected, or even knowing when to take urgent points versus big points. The biggest problem is that you don't have a good sense of which groups are alive, which are dead, and where life and death hangs in the balance. Try doing these, for starters:
http://senseis.xmp.net/?BeginnerExercises
. Don't worry, with practice you'll improve.