Go off vs go on?

So I've just seen an English paper listing many of the phrasal verbs. Two of those were "Go off: happen" and "Go on: happen". So either go off or go on means happen? Is this true? I'm a bit confused, please help me if you can. Thanks.

Solution 1:

In comments, John Lawler wrote:

Really common verbs like go, come, have, do, look, hear, take, give, bring, get, seem, and some others (note how short they are) are likely to be used in idioms. One of the most common kinds of idiom is phrasal verbs, which use really common prepositions like off, on, up, down, over, under, out, in, and some others, almost always idiomatically. Combining these two idiom chunks -- come/go plus off/on leads to any number of idiomatic senses, all of which have to be learned individually. That's the problem with idioms.

And:

The solution is to get a dictionary of phrasal verbs with examples in context. One-word glosses are not useful except as placeholders.