以下例句:
I was sleeping when he was wathcing TV.（过去进行式)
= When he was wathcing TV,I was sleeping.
Every day I read newspaper when I eat breakfast.（现在式）
= Every day when I eat brakfast,I usually read newspaper.
What do you have to do when you go to school?(现在or未来式皆可)

(1)since + 过去一个时间点 (具体的年、月、日期、钟点).
I have been here since 2002.
(2)since +一段时间+ ago
I have been here since six months ago.
(3)since +从句
I'm very sad since you left.
(4)It is +一段时间+ since从句
It is six months since I came here.
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while所接子句中的动词是进行式or单纯式,但需要是有持续性的动作或状态
例:While there is life,there is hope.(有生命就有希望)
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对现在、将来不确实事情的推测or对现在的事实相反的假设or与过去事实相反的假设皆可;
if 后面所连接的子句,可放在句首也可放在句中,但若是放在句首,则和其后子句间要用逗号分开.
例:I won't go out if you come here tomorrow.
=If you come here tomorrow,I won't go out.