The verb will in English has multiple meanings.

It can be used in the following ways:

• to use the power of one's mind to do something or to make something happen, to choose or decide, determine or decree, to exercise an act of volition: "She willed herself not to sneeze during the presentation."
• as a modal auxiliary verb

As a modal verb, will expresses the following:

• prediction of the future: "They'll be here soon." (for prediction based on fact, use Going-to future ("The result of this titration is going to be a neutral solution."))
• inferential fact of the present: "They'll no doubt be there by now." (this is equally well expressed by "They are, doubtless, there by now.")
• truths and facts: "This engine will only run on unleaded petrol."
• obligation and orders: "All students will now, please, rise." (for official orders, shall is to be used)
• habits: "He will always be willing to give you a hand whenever he can."
• annoying habits: "She will keep interrupting me when I'm trying to work."
• willingness: "I'll get some more milk if you like."
• presently formed intention, spontaneous decision: "I'll post the letter for you tomorrow." (for previously planned actions, use Going-to future ("I am going to post the letter for you tomorrow."), and for previously agreed upon actions in the future, use Present Continuous ("We are having a party next week."))
• resolve: "I will do my homework today."
• requests: "You will open the door for me."

Will as an auxiliary in the Future Perfect expresses prediction of the future or the past:

1. "I won't have finished reading the book by tomorrow."
2. "They'll have easily got there by now."

The Future Perfect tense is generally used to describe activity which has started at an undefined time in the past and will be finished by a given time in the future. It can be recognised by the words by or next. (By the time; Next weekend ...).

The past tense of will in the modal verb sense is would. Would is used in reported speech, reporting the past (with the same meanings as will), conditional clauses, wish clauses, et cetera. For example:

• I would have thought that he would have been thrilled! (or, better: "I should have thought ...") (but he wasn't)
• He promised he would have posted my letter before the post office closed. (but I don't know if he had)
• You wouldn't open the door, would you? (a polite request)
• Would you open the door for me? (a polite request)
• He said he would be late. (reported speech of He said: "I will be late.")
• You'd be so much more accepted in society if you stopped wearing these awfully smelling perfumes. (2nd type conditional; because you wear perfumes with an awful odour, you are not accepted in society; the situation is hypothetical but possible)
• If I had bought that winning ticket, I would be one hundred and twenty-five pounds richer now. (mixed conditional; I didn't buy that winning ticket, so I am not any richer now.)
• If I had stomachache, I would have told the doctor. (mixed conditional; I do not have stomachache, so I did not say anything to the doctor about it.)
• If I had had a motorcycle, I wouldn't have walked to school every day. (3rd type conditional; I did not have a motorcycle, so I walked to school.)
• He would often tell me stories when I was young. (= He used to tell me stories when I was young.)
• I wish she would stop her endless grievance. (wish clause for the present or the future, expressing annoyance; wish can only be followed by would in this case, and even so only when referring to activities performed by someone else)

For a discussion on when to use shall and when will, see Shall.

See also: future tense, going-to future.