I think "What will he say?" would simply be "Was wird er sagen?". Future perfect is basically describing the situation that will exist, after the event happens.
The confusion arises because when it comes to the verb "sagen", the translation leads to very unusual (or unnatural) English that would rarely be used.
Future perfect makes a lot more sense to an English speaker when looking at verbs like "lesen", as it's a lot more natural to effectively ask someone "When will you have finished reading the book?".
Looking at the verb "sagen" doesn't really help an English person understand German Future Perfect, because "What will he have said?" would be quite unnatural in English and very rarely used.
The use of Future Perfect with another verb such as "lesen" makes a lot more sense to an English speaker.
That would be different.
"Would" shows more of a hypothetical situation, and it's talking about something "he" already said. (e.g., "What would he have said [yesterday] [if something else had happened instead]?")
"Will" is talking about the future and is talking about something that actually is going to happen (not hypothetically). (e.g., "What will he have said [tomorrow] [before he does this other thing]?")
That means something different. "What would he say?" would use the subjunctive (Konjunktive): "Was würde er sagen?" This is the future perfect. In English, it's formed with will+have+past participle. It's used to talk about something that will be a completed action in the future (with some degree of certainty). "By the end of this month, I probably will have gained weight." I think this exercise causes problems because it's hard to say with any certainty what someone else will have said. It's easier to say what they might say or might have said. Still, when a sentence is in the future perfect in German, Duolingo wants you to translate it into the future perfect in English. Some exercises just make more sense than others.
Not really. In this case "have" is used because only one verb is conjugated, in this case it is "will" (which happens to remain unchanged). Your point about the Questions and Negatives is only partially correct. If they are formed using the helping verb "do" then it follows that "do" is the conjugated verb (third person singular "does") followed by the infinitive "have".
We don't use the future perfect very often in English, but we do have it and use it. It is constructed very similarly to German, so once you learn the pattern, it should not be difficult. Usually, a context makes its use more understandable. "By the time we get to that hill, we will have walked three miles." "By May, you will have read ten chapters." It does involve being rather positive about the future, but sometimes that's what you want. You could use a modal verb instead -- "By May, you should have read ten chapters" -- but that has a different shade of meaning. This particular example does not have an obvious context, but there could be one. You might have an unpredictable boss whose messes you are always having to clean up. He's going into a meeting and you are anticipating that he will say something that will create more problems. "What might he have said?" implies that he may have already said something he shouldn't have. So..."By the end of this meeting, what will he have said that we will have to take back?"
I think you meant to have said "they will have FOUND that".
Let's expand your two phrases to see a possible difference.
"I gave them the problem to solve two hours ago. They will have found the solution by now, I'm sure." (conjecture) "They would have found the solution earlier, if I had helped them more." (condition)
Not really. "What will he say?" would simply be "Was wird er sagen?".
Future Perfect is describing what the situation will be, after the event has taken place. To an English speaker, German Future Perfect makes very little sense when looking at the verb "sagen" because it results in English which would seldom be used and only in very specific circumstances.
It makes a lot more sense when looking at other verbs such as "lesen" or "essen", because in English it's a lot more natural to ask your friend when they will have finished reading a book, or if they are coming to visit you then you might ask them if they will have already eaten.
Not the same tense. That would be "Was hat er gesagt?" This is the future perfect tense. At some future point in time, what will have happened? It sounds more natural in a situation when you can more confidently predict the future: "By the time we get to the top of that hill, how far will we have walked?" As you can see "How far will we have walked?" is not the same as "How far did we walk?"
Just wondering....Grammatically this sentence works. But though this may be an acceptable translation in some parts of the English speaking world, I would hardly it say that it is globally used, or that such a phrase is natural , standard English. I've taught English for many years and have never come across a phrase like this in any textbook or grammar book.... nor have I heard it said by other native English speakers. More natural translation in English would be: "I wonder what he said" / What must he have said?"
The problem isn't with the tense; it's with the verb "say" and the third person singular. It's hard to say with any confidence what someone else will say or what he will have said by a future time. It would make more sense to practice this tense with a difference circumstance. "By the time we get to the top of this hill, how far will we have walked?" "By the time you are 65, how long will you have worked here?"
Sounds unnatural to me too. Maybe it's because it feels to me as if it should be part of a larger sentence to make proper sense e.g. "After the debate is over what will he have said that enraged the opposition".
"What will he say" and "What would he say" have clearly different meanings and that probably extends to "What will he have said" and "What would he have said" if you try to fit the phrases into various sentences.
I finally came up with another, perhaps contrived, context--if people were reviewing lines or stage directions or something to do with theater (or other performances)
"I will need to turn on the radio and my line is 'How many days has it been?'"
"What will he have said?"
"Right, his line before that is 'I know it's going to rain this week'"
(Scenario made up for a nonexistant play which probably deals with farming)