一般现在时态
引起让步状语从句，意思是“虽然…但是….”、“尽管…但是…”(多放于句首)
While he loves his students，he is very strict with them.虽然他爱他的学生，可是他对他们很严格
While作并列连词用，意思为“而，然而”，表对比。而but表转折。
注意用心比较体会while与but的句中用意 。
1、Some people waste food while others haven't enough. 有些人很费粮食，然而有些人却吃不饱。
2、The son was having a good meal at home，while the parents were working in the fields. 儿子在家吃好饭而父母却在田里辛勤劳作。
3、I ought to have helped her，but I never could. 我本该帮她但没能。
4、Honey is sweet，but the bee stings. 蜂蜜很甜，但蜜蜂有刺蜇人。
1、even if意思是“尽管/虽然”，引导让步状语从句，其用法类似于although，though或even though.
2、主句从句时态必须分以下两种情况：
1）如果主从句动作都尚未发生，则遵循“主将从现”原则，即主句用一般将来时，而从句用一般现在时.
如：I will go there even if it rains tomorrow.
2）如果动作已经发生，则主从句都用一般过去时.
如：He died even if he was well treated by some experts.
3、注：以上时态只是一般的情况，不能排除特殊情况.
如：I would never marry you even if you were a millionaire.虚拟语气
