be in doubt
Does this make sense?

"I do not want you to be in doubt when doing this exercise"

Question

be in doubt
Does this make sense?

"I do not want you to be in doubt when doing this exercise"

Answer

Yes. It just means "I want you to be sure" just like what Ruthi had mentioned.

Answer

It doesn't sound right. In doubt about what?

Do you mean something like:  'I want you to be clear about what's expected of you when you do this exercise' ?

Answer

Yes it is the same as saying "I want to be sure"