Only use your car when absolutely necessary.

【翻译】绝对必要时才使用你的车。

主句为祈使句，从句为 when 之后省略了 it is 的时间从句。

这句话的意思是：You can use your car only when it it absolutely necessary.

【说明】only 的这种用法，也属于“否定转移”，位置虽然在动词之前，但在实际翻译和理解时，是修饰后面的词组或从句。

Only use your car when absolutely necessary. = Use your car only when absolutely necessary.只有在绝对必要时才使用你的车。

Only use capital letters for special emphasis. = Use capital letters only for special emphasis.只有在特别强调的时候才用大写字母。

We only use detergent liquids in our restaurant. = We use detergent liquids only in our restaurant. 我们只在饭店用清洁剂。

We only use these plates on special occasions. = We use these plates only on special occasions.  我们只在特别场合才用这些碟子。

You should only use this door in an emergency. = You should use this door only in an emergency.  只有在紧急情况下才能使用这扇门。

I only do it for amusement, not for any serious purpose. = I do it only for amusement, not for any serious purpose.  我只不过是做着玩而已，并无正经目的。

He can only do his homework in the way I told him. = He can do his homework only in the way I told him.他只会按我告诉他的方法做作业。