求if与weather的区别

应该是“if”和“whether”的区别。

weather 是“天气”；in case 是“ 假使；免得，以防；也许”；unless 是“除非”。whether与if作“是否”讲，在用法上有相同之处，也有不同之处。

①，相同之处1.whether和if都能引导宾语从句，常置于see,ask,learn,tell,wonder,doubt,find out等动词之后。He asked me whether/if I could help him.I want to know whether/if he lives there.2.whether和if都可用于it作形式主语的从句中。It's uncertain whether/if he'll come this evening.3.在be uncertain/doubtful之后，常用whether引导宾语从句，有时也可以用if。We are uncertain whether/if Mary will come.②，不同之处1.whether可置于句首引导主语从句，而if不能。Whether the 2000 Olympic Games will be in Beijing is not known yet.2.whether可用在介词之后，引导宾语从句，而If则不能。Success depends on whether we make enough effort.3.whether可引导表语从句，if则不能。The question is whether it is not worth doing.4.在名词之后引导同位语从句时，用whether而不用if。The question whether the students answered correctly should be decided by the teacher.5.whether后可接不定式，而if不能。Please tell us whether to go or stay here.6.在有些动词（如discuss）之后，只能用whether，而不用if引导宾语从句。We discussed whether we should close the shop.7.在引导否定含义的宾语从句时，只能用if，而不用whether。He asked me if I hadn't finished my work.