How to use would in different expression in this way?
You would have to do/say that.
You have to do/say that.
Why would is used here?
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Would has a simple, non-modal use as the past tense of will. You can see this when tenses get backshifted in indirect discourse. The direct speech
He said, "I will go."
He said that he would go.
But the word also has a host of modal uses, that is, uses that reflect different aspects of meaning beyond person, time, and number. These are largely idiomatic, which means that they are not amenable to analysis. They have just become fixed in the language over the course of the millennium that the the word has been in the language. Here are a few usages:
It is not an easy question to answer and would as a modal verb has a lot of meanings. You would need to look up the dictionary and get yourself familiarized with all those definitions and usage examples. In your example, it means No. 5 in the link:
Expressing a conjecture, opinion, or hope
The main difference between "you have to do that" and "you would have to do that" is the latter is more polite and less strong in terms of requesting the other party (you) to do that. In other words, the latter shows less conviction of the speaker in terms of why the action should be done. The latter could be rephrased to:
I think you have to do that. In my opinion, you have to do that.
Would also could be used to express a desire or polite request. It will largely depend on the context.
[Oxford Online Dictionary]