What is the difference between 'you have to say that' and 'you would have to say that'?

Question

How to use would in different expression in this way?

For example:

1. You would have to do/say that.

2. You have to do/say that.

Why would is used here?

Answer 1

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."

becomes

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:

• Willingness: They asked us whether we would join them for dinner.
• Consent: If you would only stop talking, I will explain.
• Preference: I would rather not. I would love to come to the party.
• Accustomed to: He would often come home drunk.
• Capability: I thought I had the right key, but it would not work.
• Persistence: I told him to stop, but he would not be deterred.
• Determination: Be wary of those who would impose their tastes on others.
• Intention (when backshifted in indirect discourse): He said, "I will pay the bill myself" -> He said that he would pay the bill himself. (In the conclusion matching a conditional subjunctive): If you were to ask me, I would say no.
• Inevitability, compulsion (as an alternative to should): I would know that face anywhere.
• Refusal (in the negative): No matter what Obama proposed, the Republicans would have none of it.
• Desire (with subject-verb inversion): Would that God grant my prayers.
• In an expression of a wish, as the verb in the clausal object: I hoped that he would arrive today.
• Past report of future action: The Puritan's chief concern was that somebody, somewhere would enjoy himself.
• Present indirect report of future action: He said, "I will go." -> He said that he would go.
• In 2nd and 3rd person indirect discourse, as a substitute for 1st person should: I said, "I should have the report for you tomorrow" -> He said that he would have the report tomorrow
• Emphasis In rhetorical questions: Who would have thought her capable of murder?
• Presumption or probable past state: They were in the same class, so they would have know each other.
• Necessity: He grew so angry, it seemed he would explode.
• Uncertainty: So it would seem. So I would think.
• Softening of a declarative statement: I think that he has stopped gambling -> I would think that he has stopped gambling.
• Softening of an imperative statement: Will you set the table -> Would you set the table.
• Expressing possibility in the conclusion matching a conditional subjunctive: If he weren't a biter, then your kid would be in daycare.
• Expressing advice in the conclusion matching a conditional subjunctive, which might be understood but not stated: I wouldn't do that [if I were you].

Answer 2

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]