if和unless从句的时态

　　You can come with us if you want to.

　　如果你愿意可以跟我们一起来。

　　Put your hand up if you need more paper.

　　若有人还要纸，请举手。

　　Unless I'm mistaken, she was back at work yesterday.

　　除非是我记错了，她是昨天回来上班的。

　　Unless we start making some sales, we're done for.

　　如果我们还卖不出去，那我们就完了。

　　I sleep with the window open unless it's really cold.

　　天气若不很冷，我总开着窗户睡觉。