when it is not practical to use the CAT
Yeah, that is fine.
You should use "are".
You should use did.
you should use meters
You should use your treadmill on a regular basis. You should use it every day if possible. You should use the treadmill for twenty to thirty minutes.
you should use you all
Why should you improve your ability to use the library?
There are number of different kinds of gas you should use. The gas you should use depends on the situation.
You should not. You should use centimetres.
we should use had if the sentence is in past tence
if we made mistake