“will, would. . . .It is a question of clarity–of telling the difference between what may happen and what will happen. If you write, “The plan will cost $400 million,” you are expressing a certainty. The plan either has been adopted or is certain to be adopted. If you write, “The plan would cost $400 million,” the statement is clearly suppositional. It is saying only that if the plan were adopted, it would cost $400 million.”
-Bill Bryson, Bryson’s Dictionary of Troublesome Words
I still get this one wrong sometimes, like:
“I wouldn’t presume to know enough about English to start a writing blog.”
Really? I wouldn’t? I wouldn’t if what were the case? Bryson is good to remind us that “will” is indicative (“expressing a simple statement of fact”) and “would” is subjunctive (“expressing what is imagined or wished or possible”). So in the above sense, there needs to be an “if” somewhere, visible or implied, for “would” to fly.
Keep in mind that “if” doesn’t always require “would”, since “if” can put us in the conditional as well as the subjunctive. When something is definitely going to happen, stick with the definite “will”. Like:
“I will burst into tears if no one comments on this post.”