How can I understand what function 是 has in this sentence?
it is very optional. By feeling I would translate it with "is it that.. " which honestly is 90% the same as "did.. " which just translates to 她用美图应用了吗？
Without 是 is also marked as correct now
用美图应用了吗 will be enough.
是 is putting an emphasis here
Why is it 真大 and not 太大了?
Wouldn't 太大了 mean "too big" , while 真大 means "very big" or "so big" ?
It's le, not liao, you tools!
In this case, it should be pronounced le instead of liao.
I think, this not-a-must 是 kind of became a conjunction in this kind of sentence and help it sound smoother, especially when you're guessing something you're not 100% sure of. 是。。。用了美图应用吗？