If seeing the photograph of my friend now can remind me of my friend, it's got to be because I met my friend before.

如果看到我的朋友的照片,就能想起他/她本人,这是因为我以前见过我的朋友

耶鲁公开课 - 死亡课程节选

you can call your friend and ask them when they'll be free by using a "when" question.

你可以打电话给他们，用“When”问句来询问他们什么时候有空。

When 课堂 - SpeakingMax英语口语达人