Present perfect for past events

The present perfect tense is used for past events when the exact time is not mentioned either because it is not known or it is not important. The emphasis is on the fact that the events happened before the time of speaking:

I've been to China.
I've seen that film before.

But if we say when something happened, we use the past simple:

I went to China in 2011.
I already saw that film last week.

We also use the past simple if we ask when something happened in the past:

"Have you ever been to China?"
"Yes, I have."
"When did you go there?"

"Have you seen that film before?"
"Yes, I have."
"When did you see it?"