when I saw him yesterday,I forgot when meeting him before 对吗

不太对哎。。你想表达的意思是什么？

如果是当我昨天遇到他的时候，我忘了我曾经什么时候见过他。

正确的表达应该是:

When I saw him yesterday, I couldn't remember when I had met him before.

希望帮到你呀。

When i saw him yesterday,I forgot the day on which i met him ever.

When I saw him yesterday, I couldn't remember when I had met him before.
如果是当我昨天遇到他的时候，我忘了我曾经什么时候见过他。

希望帮到你呀!(*^__^*) 嘻嘻……

不对哦

这样：
when I saw him yesterday,I forgot when I had met him before.