Asking questions helps you maintain control. For instance, when you are waiting for a safe gap to turn left, you ask the student “After what vehicle do you think it will be safe to go?” If the answer you get does not leave you a large enough gap, you can correct your student by saying “Let’s wait until after that red truck (or wherever you determine the safer gap is) instead”. While waiting for the opening you can prompt your student to glance up the new path for the new target, and to continue to wait with the wheels straight.
Also, at an unprotected light make sure that you have entered the intersection on the green light and can comfortably wait in the intersection. You want to be waiting with your vehicle’s wheels straight and well short of the turning point while you are looking for your gap.
Why? Well, as you recall, you wait with your wheels straight so that if you are tagged from the rear, you will not be pushed into oncoming traffic.
You wait well short of the turning point so that, when it’s safe to turn you can begin to move forward first. This gives you a little momentum and allows you to clear the intersection in less time (because it takes less time to clear the intersection if you are moving than if you are coming from a stop).
You wait in the intersection because if the light changes while you are in the intersection, you can legally complete your turn when it is safe to do. Being in the intersection also makes you more visible to cross traffic.
As your student makes the left turn, you count out loud the number of seconds it takes to complete the turn (clear the intersection). This will give the student a more accurate assessment as to how much of a gap in traffic is needed to safely complete the turn.
The speed of the turn itself will probably be a teaching point. If your student is anxious, he/she may go too fast. If your student is a bit timid, he/she may be reluctant to pick up his/her speed enough to clear the intersection promptly. It’s even possible for the student to do well on this in the parking lot exercises and even in the neighborhood, and yet have a less than perfect response when in light traffic. With sufficient exposure to these turns in traffic your student will become more comfortable. Learn from each turn and strive to improve each time. Progress will come.