This one really got me... But I found a helpful trick:

ONLY IF states a NECESSARY condition

Necesary conditions are always the superset.

you can rewrite 'only if' statements to 'if then' statements

Ex. Students are cited as "late" only if they arrive more than five minutes past the last ring of the homeroom bell.

This is the same as:

If Students are cited as "late" then they arrive more than five minutes past the last ring of the homeroom bell.

these are the same but the second one makes the necessary condition much clearer.

Late --> 5+