Question

It's very kind of you _____(say) so. 并说明理由，在线等、

Answer 1

It's very kind of you to say so.
be kind of somebody 后要接to do something.这是一种固定用法。
例句：
It's verykindof you to help me.
谢谢您好心帮助我。
It's very kind of you to tell me.
你来告诉我真的太好了。
It's very kind of you to help me with these problems.
你帮我解决了这么多难题，真是太好了。

Answer 2

这里可以填两个答案，to say或者saying。
It's very kind of you to say so.表示将要说的。

It's very kind of you saying so.表示已经说过的。

另外对于后者，It's very kind of you (by /for)saying so.直译的话就是:通过(因为)你这么说,我觉得你很好，所以是省略了介词，应该用saying.

Answer 3

It's very kind of you to say so.
动词不定式to say so 是句子的主语，动词不定式作主语常用it 来代替。
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