The students can ask me or the TA abut the exercises. We prefer to see what the student has attempted, and find the exact spot where the student made the mistake. It takes more time, but it's more helpful than a nice ideal solution.
We also may recommend to make another exercise from the official list and come back in 10 minutes with a solution attempt, or make some custom exercise on the spot that is about the same subject. (Exercises about derivatives are easy to invent, integrals and linear algebra is harder.)
One of the useful trick is to write an optional homework exercise in the blackboard. I get a lot more of answers than just selecting an exercise from the official list. Sometimes it's just an exercise from the list with different number and sometime it has an intermediate step to make it easier. But writing it in the blackboard increase the chance the students will write it down.
Another trick is to write an old midterm and then go to each desk to talk to the students about what they are doing. We usually write the solution at the end of the class, and have some discussion about it, but the useful part is the discussion with each student (or small group of 2-3 students).