(c)if+一般现在时+must,should或任何表示命令、请求或劝告的形式:
If you want to lose weight you must/should eat less bread.
如果你想减轻体重,你必须/应该少吃面包。
If you want to lose weight you had better eat less bread.
如果你想减轻体重,最好少吃面包。
If you want to lose weight eat less bread.
如果你想减轻体重,少吃面包。
If you see Tom tomorrow could you ask him to ring me?
如果你明天见到汤姆,你能让他给我打个电话吗?
(d)if+一般现在时+一般现在时
if+两个一般现在时可以用来表示自动产生或照例会产生的结果:
If you heat ice it turns to water.
如果把冰加热,它就化成水。(也可以用will turn。)
If there is a shortage of any product prices of that product go up.
如果某种产品短缺,那种产品的价格就上涨。
(e)当if用来表示 as/since时,主句中就可能使用多种不同时态:
Ann hates London.
If she hates it why does she live there?/she ought to move out.
安讨厌伦敦。
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2016-02-29
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