1.They're speaking to each other.2.We must all help each other.第一个句子加了to,因为speak to是个词组,为什么第二个句子help后面不加个in,构成一个词组,

1.They're speaking to each other.2.We must all help each other.
第一个句子加了to,因为speak to是个词组,为什么第二个句子help后面不加个in,构成一个词组,

1、speak 有两种用法.作为及物动词,后面只能跟语言（如 speak English 讲英语）或话语（speak the truth 说实话,speak words of sympathy 说一些同情的话）之类的名词作宾语；如果要表达“对某人说话”,speak 则是不及物动词,必须和表示“向”之意的介词 to 以后才能带表示人的名词或代词作宾语.
2、help 也有两种用法
1）及物动词,后面可以直接跟表示人的帮助对象作宾语.help in sb.或者 help sb.in 中的 in 是副词,意思是“把某人搀扶进来”,如：
He helped in his grandmother.（或者 He helped his grandmother in.） 他搀扶着祖母进屋.
—— 如果宾语是代词,宾语只能放在副词 in 前.如：
Please help him in.把他搀扶进来.
They helped each other in.他们互相搀扶着进来.
2）不及物动词,后面加介词 in 表示的是“帮助干某事”或“ 在…方面给予帮助”,其宾语是事物,如：
They often help in the farm work in our village.他们常常帮助我们村干农活.
He helped in the recent affair.他在近期的事务中给予帮助.

speak是不及物动词，后接宾语要用介词过渡，
help是及物动词，直接加宾语，不用加介词就可以构成动宾结构

speak 是不及物动词要加介词才加名词，而help是及物动词直接加名的。：互相帮助：

help是及物动词，直接加宾语，不用加介词。
help each other 的意思是互相帮助
希望对你有帮助。

不及物动词后接宾语要用适当的介词
如：look at the blackboard.
he is looking for his father.
They are talking about the movie star.

不及物动词后接不宾语可不用介词
如：Look! He is coming.
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不及物动词后接宾语要用适当的介词
如：look at the blackboard.
he is looking for his father.
They are talking about the movie star.

不及物动词后接不宾语可不用介词
如：Look! He is coming.
Don't talk!
及物动词后可直接加宾语
He always helps others.

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