What's the meaning of "when I fall in love, it will be forever"?

Question

If someone says "when I fall in love, it will be forever", which of the following does that mean?

1. The speaker is pointing out the fact that they have already fallen in love and this is something that will last for them forever.
2. The speaker hasn't fallen in love yet but if in future they do fall in love, then it will last for them forever.

Answer 1

In temporal or conditional clauses in English (those introduced by words like when or if) we mark past meaning, but not future. So

When I saw him

is clearly past, but

When I see him

could be present or future. (The only kind of present it could be is a habitual, as in

When I see him, we always talk for too long.)

But

• When I will see him

Is never idiomatic in English, and is arguably not grammatical.

So when the temporal clause is non-past, it is the main clause that marks whether it is to be taken as future or not.

So in the case you are asking about,

When I fall in love, it will be forever.

the main clause is marked as future, so the temporal clause must also be future, i.e. your meaning 2.

To get your meaning 1, the temporal clause would have to be past:

When I fell in love, it was forever or When I fell in love, it would be forever

(using "would" as a "future in the past").