In addition to has already been posted, you might want to look at your hands, and their position at the top. If you are right handed, you might want to make sure your right hand (wrist) is under the shaft. By that I mean bent backwards. Sometimes for right handers if the right hand is not under the shaft at the top, the right hand will over power the the weaker left hand at the start of the down swing, which can throw the club head outside the line of the swing. It can also cause the club face to be open at the top. With the right hand (wrist) under the shaft, the line formed by the back of the left hand, and it's wrist will be pretty straight.

I posted sometimes, because some players will not have the right hand (wrist) under the shaft, but can manipulate their swing to correct this on the way down. GJS