It's a bit strange, but here's the rule: if you can divide the time signature by 3/2 (and not get 1 anywhere), it's the same as the result, but in triplets. So

It's a bit strange, but here's the rule: if you can divide the time signature by 3/2 (and not get 1 anywhere), it's the same as the result, but in triplets. So 6/8 is a variant of 2/4, with 2 main beats in the bar. 3/4 has 3 beats to a bar. Or in quavers, 6/8 is Da du du Da du du, whilst 3/4 is Da du Da du Da du.