[Nightedge]

Quote susan53

Possible yes, but it would need a convincing context to show that I didn't know when the specific decision moment (which is an event with a specific time) was .

Do you mean 'decided', the simple past, is possible even when since is used to indicate the time? If yes, could you think of an example? I couldn't, because the simple past with decide is used to talk about a non-continuous act, which means that if decided, not have/has decided, is used, it must be talking about a particular moment, not something that since can refer to.

[susan53]

Not in the same clause, no. Look at the examples above. The verb in the clause with since is always in perfect aspect.

What I'm saying is that it would be unusual to use the verb decide together with since, as, generally, you know when the decision was made. I've given a couple of examples when it might be possible to say eg since then, indicating that I don't know exactly when the decision was taken (or if he's actually done anything about it. But generally a decision happens at a specific moment - which you know about - and it's what you decided to do that happens afterwards, as in the last example.

As always, you must consider the context. language use doesn't happen in a vacuum. It's affected by the situation. The problem with your examples is that the context is never provided. If you give one, then you'll find it's much easier to understand why certain forms are used rather than others. That's why my answers always add one.

[Nightedge]

Excellent answer. Thank you.