Hi Grace
I'm working on this too, trying to revise what I learned 20-odd years ago! If I have remembered this correctly, you have four types of cadence which you may have to identify: perfect (V -I);plagal (IV - I); imperfect (I - V) (this must be what squinchy calls a half cadence); interrupted (V -VI).
The perfect and plagal ones will sound "finished", as Squinchy says. I can't do better than Squinchy's description of the plagal one as sounding like "Amen".
The imperfect and interrupted ones will sound "unfinished" - as if the tune needs to go on a bit further to reach a satisfactory conclusion. The Yankee Doodle example for the imperfect cadence is a good one.
As for the interrupted cadence, it goes from V - VI. If you're in a major key, the V-chord will be major, while the VI chord will be minor. So if your tune changes modality, you're likely to be listening to an interrupted cadence.
That's the cadences bit taken care of, which is part of the question. For the rest of the task, you have to identify up to four chords in the progression (rather than cadences). Best thing to do to get yourself used to the sounds, is to play a scale of C major, in triads rather than single notes. So you'd play CEG, DFA, EGB etc. You'll notice that some of these are major (I,IV,V, VIII) and some are minor (II,III, VI).
You need to practice keeping the key chord in mind, and play the other triads randomly, to get used to the sounds.
You only have to bother about I, II, IV, V (and V-7) and VI for the exam, I think - is this the same as UK Grade 8? Okay, this makes it sound easy, and it's more difficult in an exam with an unfamiliar piece of music. But at least if you're used to the sounds, you can make a decent stab at it. If you're in a major key and the chord is a minor one, you'll know that it's more likely to be II or VI, so you've got a 50% percent chance of getting it right, which is better than a 20% chance.
This is Kathryn's trial and error approach to harmony, strictly for use in times of urgent need. If I manage to work out a better system before my own Grade 8 exam in a few weeks, I'll let you know. If anyone else has a better, foolproof method, please post!
kathryn