I've got a Grade 5 Theory exam coming up soon and in the practise papers I have been really struggling with the questions where you have to put in the bar lines. When there are loads of demisemiquavers etc I get really confused. Any tips/ help?
Thanks

You have to figure out the value of each note and work it out with the time signature.

Look at some of these vjdeos for info on note values and how to work out the time in a measure --

Music Theory

It all boils down to math and counting (addition).