if引导时间状语从句，当if做“(每)当”或“无论何时”解而不含有条件之义时，if从句中的时态与主句中的时态相同。相当于when, whenever。
例如：
If metal gets hot it expands.
（当）金属受热就膨胀。
She glares at me if I go near her desk.
我一走近她的办公桌，她就对我瞪眼。
If you mix yellow and blue，you get green.
你将黄色与蓝色混合，便会得到绿色。
If she wants the servant，she rings the bell.
每当她需要仆人时，她便按铃。
If our children are doing their homework, my husband and I always try to be quiet.
每当孩子们做家庭作业时，我和我的丈夫总是尽量保持安静。
If a writer does not go deep among the masses, he cannot expect to turn out good works.
当作家而不深入群众，就不会写出好作品来。
If ( one was) overwhelmed by passion, it could lead to serious blunders.
当感情完全淹没理智时，就可能铸成大错。
He gave great encouragement to his students, especially if their passions happened to coincide with his own.
他给予学生莫大的鼓励，尤其是当他们与自己志趣相投时.