I would say: She remembered everything as if it had happened yesterday The main reason is because “had happened” emphasizes the concept that it’s all hypothetical; it didn’t really happen yesterday, and that’s quite clear from the “had happened”. However, I believe the vast majority of people would naturally say “as if it happened yesterday”; for this reson, I can see why your tutor told you the first option is the most accurate one. I wouldn’t focus too much on grammar-based explanation in this example, because you might read good arguments for both options. I would go for the most natural sounding expression, as you can’t really say there is a definitely right nor a definitely wrong option (although some might say there is…😅) P.S. definitely "it"; not "they". 😁

Lots of answers already. The use of 'they' is a bit odd. I would say 'it' ...unless there is a very good reason to say 'they'... and only the context would tell us! I much prefer number 2.. had happened. Better style, to me. BUT, and it's a BIG but, we need some context and dates. And if the day of remembering was, say, a day in 1942, and 'yesterday' was indeed just yesterday (!) HAD happened yesterday would work fine still. I have no idea why your tutor would say 'yesterday' is not a sign for a past point. It is definitely past. Is this some sort of confusion with 'past perfect is only used as a past before a past' ??? It often is, but not always (or at least, the TWO pasts are not necessarily in the same sentence). Look, we really need MORE context and some dates to be sure - because there is more than one meaning here!

I wonder why your teacher use *they* here? Since it is about and incident or action taking place. "She remembered everything *as if it happened yesterday* ( or as if it happened moments ago )"

The sentences use subjunctive mood. Subjunctive mood may look like past tense (except for "to be") but it is actually something different. Both sentences are good once you change "they" to "it". There is a third way that only works with subjunctive mood: "She remembered everything, as had it happened yesterday." When you say it this way, "if" is no longer needed. In fact, you must drop the "if".

Any word that starts with 'every' is singular (everything, everyone, everybody, everywhere). 1 and 2 are both correct, 1 is easier to say, but 2 is more precise. She remembered everything as if it happened yesterday. She remembered everything as if it had happened yesterday. We also have a phrase that we say more often, that is very similar to this: She remembered it like it was (only) yesterday. I remember it like it was (only) yesterday.