Straight Left Arm | Golf Monthly
Club golfer unless it's natural is not going to have that range of mobility in the shoulder joints, muscles across the back, the abdomen to support that big a turn, or the flexibility in the hamstrings.
Having a left arm that collapses at the left elbow so very bent is a no no, having a relaxed left arm that is soft but virtually straight is fine, can be a slight, very slight give so it's nearly straight all the ways through the motion is okay as it still gives you a 'fixed' radius for the hand path.
Important the wrists/hands set in the right ways as that still gives you a 90º angle between left arm & shaft when the left arm is 1st horizontal to the ground, then as long as the arms & really connected to the body turn & they all stop together even if thats a 3/4 length swing, out can still move it out there.
What confuses the issue some if folks lift their arms away from the body turn in an attempt to get a bigger swing, may feel more powerful but in actuality it isn't, as the lift away at top of the arms, or the collapse of the elbows to make the swing longer, or both just disconnect the arms from the body turn so you have a power leak & not as much speed through impact. Also either of these things or both will give you real timing issues trying to re-connect the arms to body turn on the way down to impact, so lots of contact issues too.
If flexibility an issue try to keep as straight a left arm as possible, make sure the wrists set so you have that 90º arm shaft angle when the left arm is horizontal but only turn as far as you can keep arms & body turn properly connected.
You can always to just move the trail foot back away from the ball target line a little, so you are a little closed & make sure the right hip turns some & doesn't sway laterally to the right, as that will make it a little easier to load the turn to the top, as long as you then have the left arm shaft angle at 90º & your connected to the body turn you won't really lose too much distance, might even find you've gained some.