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". 😁