"won't" vs. "wouldn't"

Question

Are these two words interchangeable? How do you know when to use one or the other?

For some sentences it is easy to know which one to use, but not for others. The type of sentences that are difficult are those that begin with "if" or "should."

Here's some example sentences:

• Should you ever break up with your girlfriend, wouldn't you become lonely?
• Should you lose this, won't finding another one be difficult?

Or any kind of sentence that is like "Should ___, wouldn't/won't ____?"

Answer 1

Contrary to what you seem to think, wouldn't and won't are almost never interchangeable.

The simple negative won't is used for future negative actions or for refusals.

• I won't go to the store tomorrow if it's raining. (Future negative.)
• I won't go to the dance with you. (Refusal.)

The negative wouldn't is used for counterfactual statements, and for future statements embedded in a past-tense narrative.

• I wouldn't shout if I were you. (Counterfactual)
• He said he wouldn't like it. (Future embedded in past narrative.)

In every case here, replacing won't with wouldn't results in something either ungrammatical, or it changes the meaning of the sentence.

Edit: An additional requirement for will/would is tense concord, which means that subordinate verbs in a complex or compound sentence must agree in tense with the main verbs. This applies to the two halves of an if/then construction, as well as to verbs in relative clauses. For this purpose, will is considered to be present tense, and would is past tense. So you see things like:

• He will be dead if he goes to the store.

• He would be dead if he went to the store.

• He says he will open the envelope.

• He said he would open the envelope.

In this case, the distinction between will/would doesn't carry any semantic weight, but is required by English grammar. Swapping will and would in any of the preceding sentences results in an ungrammatical utterance.

Answer 2

As Colin mentions, in this context would implies a hypothetical situation, whereas will implies intention or (likely) prediction.

So to take one of the examples:

If* you ever break up with your girlfriend, won't you become lonely?

...implies a greater likelihood of the break-up occurring than:

If you ever break up with your girlfriend, wouldn't you become lonely?

In the latter case, because we are talking about a hypothetical occurrence, the first part of the sentence could well be changed to be subjunctive:

If you were to ever break up with your girlfriend, wouldn't you become lonely?

...in which case wouldn't has to be used instead of won't.

(* Note: I've used if rather than should here, as should sounds rather old-fashioned when used in this context, to my ears at least.)

Answer 3

"Will" and its negative "won't" are about future intention or prediction.

"Would" and its negative "wouldn't" have a range of meanings, but the relevant one here is about hypothetical intention or prediction.

So "I won't go" is a statement of my actual intention.

"I wouldn't go" is saying something about my intention in some hypothetical situation.

I'm sure there are contexts when both can be used, but I think there will still be a difference in meaning.

Answer 4

As Kieren has commented, won't is short for will not whereas wouldn't is short for would not. They're not interchangeable. e.g. you wouldn't write I won't do that if I were you, you'd write I wouldn't do that if I were you. If you're not going to be at a party you might write I won't be there and not I wouldn't be there.

Answer 5

From Grammar in use — intermediate, 3rd Edition, pp. 40–41, Cambridge University Press, 2009:

You can use won't to say that somebody refuses to do something:

I've tried to give her advice, but she won't listen.

The car won't start. (=the car refuses to start)

Ibidem, pp. 68–69:

Somebody wouldn't do something = he/she refused to do it:

I tried to warn him, but he wouldn't listen to me. (= he refused to listen)

The car wouldn't start. (= it refused to start)

Answer 6

Won't (Will [not]) versus Would

The question centers on the construct called "verb mood," three in English of which are: indicative (a matter of fact), imperative (a matter of necessity), subjunctive (a matter of possiblity). [See http://englishplus.com/grammar/00000347.htm, and links within.]

The mood "breakdown" (pun intended) for will:

• Indicative: will
• Imperative: shall, will (non-standard; contextually or with inflection)
• Subjunctive: might, would

Wherever any mood form is shared (two or more forms appear together as a singular mood) those are interchangeable when intended meaning otherwise is not lost.

Here you see that won't (negation or will in Indicative or Imperative mood) never shares mood with would.

So the answer is, no.

If a thing would..., that is only a matter of possiblity, subject to doubt. If a thing will or shall..., neither is open to doubt.

(Will that answer your question? Or wouldn't it?)