When should I use "Would", "Would have", "Will", and "Will have"?

Question

I hope someone, once and for all, can clarify (with examples) the difference in usage of will vs. would vs. would have vs. will have.

Answer 1

This question is quite broad, and I find it quite hard to come up with an answer that is comprehensive yet succinct, technically impeccable yet easy to understand. At the risk of failing miserably, I'll give it a try nonetheless.

Will, would, and have are auxiliary verbs used to form different tenses.

Will is used to form Future Simple, to describe something that takes place in the future.

• I will be there.
  ⇒ I am not there. But I plan to be there at some point in the future.
• I will do that.
  ⇒ I am not doing it right now. But I plan to do it at some point in the future.

Will have is used to form Future Perfect, to describe something that not simply takes place in the future, but is completed ("perfected") at some reference point in the future (which is usually specified).

• I will have done that by the end of the week.
  ⇒ I will do that, and at the reference point (end of the week), my action will be over; I will be able to say "I have done it".

Would is a past-tense form of will. If you are writing about past events, you can use it to indicate something that was in the future at that point in time, but is not necessarily in the future right now. In other words, you use would to preserve the future aspect when talking about the past.

• She said that she would visit me.
  ⇒ This is the same as: She said, "I will visit you". It's called making verb tenses "agree".
• I thought she would have visited me by now, but she hasn't.

Would is also used as a modal verb to indicate a conditional or subjunctive mood, or to "soften" what is being said:

• I would love to see that movie.
• If I had a hammer, I would use it as often as possible.
• Would you give me that book, please?

I realize that I have omitted a few things (say, Future Perfect Continuous). Again, that was a conscious decision for the sake of simplicity.

Answer 2

From Commonly Misused Terms and Phrases

• “Will” suggests strong probability or future likelihood.
• "Would” implies the same, but is typically used when the probability is more hypothetical.

The article Will you or would you? gives a few examples.

Will have or would have you might find useful.

Answer 3

"Would" has two meanings and I'm not sure which you mean, but one is a modal verb and conditional.

I would wash the car if the water was working.

The other one is past tense and basically just means "used to"

He would plow the field with team of horses in just days.

"Will" is for certain.

The next year will be 2011.

Adding "have" to the end just makes them the perfect tense, which kinda references the past too. I'm not sure it works with the conditional type of would, but I could be wrong.

• He would have taken me, but he had a flat tire.
• It will have been gone for a whole month by tomorrow.