can someone correct this sentence for me plz? thanz a lot

AnonymousAt that time, I discovered [that] what people said say about how time flies fast, is true.

why is there a comma before IS?

No good reason. You've interrupted "what people said is true" with "about how time flies fast." It doesn't seem necessary to actually set it off, but, it seems like when you read it, and especially when you speak it, you need to indicate where the joint comes. If it were a little shorter, the brain (my brain in particular) could easily grasp it without the comma: " . . . what people say

oh okie thanz my friend

OK, I was wondering as well.. can I put it like..?
"At that time, I've discovered, what people had been saying, on how fast the time gose/flies.."

No - At that time requires the simple past, and don't use a comma after discovered.

At that time, I discovered that what people had been saying about how time flies is true.

But try something like this: It was then that I discovered the truth of what people have always said: Time does fly!

awesome, thank you )

ivo