在英语中，如果你想表示“当时你做什么”，可以使用以下表达方式：

"What were you doing at the time?"（当时你在做什么？）
"What were you up to at that time?"（当时你在做什么？）
"What did you do then?"（那时你做了什么？）

这些表达方式均使用了动词"do"或"be up to"来描述某人在某个时间段内在做什么。

举例来说，如果你想问别人当时在做什么，你可以说：

"What were you doing at the time when the accident happened?"（当事故发生时你在做什么？）
"What were you up to when I called you yesterday?"（昨天我打电话给你时你在做什么？）
"What did you do then, when you reapzed you had left your wallet at home?"（当你发现你把钱包忘在家里时你做了什么？）

希望以上回答能帮到你！