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I sometimes come across questions in conversations where the word "does" is used in a situation where "could" would normally be used. For example if someone asks the following about someone else's car:

Does it run on gasoline?

In a general context using "does" in a question like this would be understood as asking if gasoline is the sole or primary fuel used by the car. For example if the asker doesn’t know what type of car it is and is asking if the car is gasoline powered or diesel fuel powered (and nowadays we can include electric powered).

In a situation where the asker already knows that the car is a diesel and uses diesel fuel, but they are wondering if gasoline can also be used, they would normally not state the question using “does” but would instead use “could”:

Could it run on gasoline?

If they were to use “does” in this case it could cause the hearer to mistakenly assume that the asker doesn’t know that it's a diesel, or that diesels use diesel fuel, leading to a more basic answer than what the asker is looking for.

However I have sometimes seen situations where the asker uses “does” instead of “could”, but because of the context the hearer correctly interprets that the question really means “could it”. This can happen for example where it has been already established in the conversation that diesels use diesel fuel, so the hearer realizes that a more specific question is being asked. Or perhaps if the question is a follow-on to a preceding statement, for example:

I see that you are filling it with diesel fuel. Does it run on gasoline?

I suspect that this is not semantically correct, even if this type of usage of the word “does” seems to work and at times can communicate the point effectively, at least in these specific contexts. Normally if "does" were being used in this type of scenario it would be used with additional modifiers, for example:

Does it also run on gasoline?

However it is common in casual conversation to omit words for brevity, especially if the meaning still comes across. Of course that can often result in improper syntax or semantics, but I don’t know if the semantics line is actually being crossed in these specific situations that I am referring to, since it doesn't at least from what I have observed seem to create any misunderstanding.

There does seem to be a nuance between using “does” and the more correct “could” in these situations that might explain why this sometimes happens. “Could it run on gasoline?” seems to acknowledge that running a diesel car on gasoline would be very undesirable or perhaps harmful to the engine, but is asking if it’s possible in case of necessity, for example if the question was asked during a hurricane evacuation. Whereas “Does it run on gasoline?” seems to imply that while diesel may be the primary fuel, perhaps gasoline is an alternative, albeit at maybe lower efficiency or some other less catastrophic result. Perhaps this is the reason “does” is sometimes used in these situations instead of “could”. But that's just a theory and is not part of the question, which is more about the semantic correctness of using "does" in this manner.

EDIT: The above question was rewritten for clarity. It should be noted that some of the answers and comments below were written based on the original question text. I apologize for the increased length of the edited version, however the shorter version didn't seem to illustrate clearly enough the nuance of the semantics involved.

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    'Does it run on gasoline?' means 'Does it use gasoline as fuel?' / 'Could it run on gasoline?' would only be used if it is known that the usual fuel is diesel, say. Commented May 17 at 16:25
  • We need to distinguish between the entailed meaning and the way that entailed meaning can be wielded in practical situations. The same question can be understood differently in different situations. I have a little inverter generator that says "Dual Fuel" and it can run on propane or gasoline. Does it run on gasonline? might have the practical in-context meaning "Is gasoline one of the fuels it can use?" Maybe it takes diesel and propane or gasoline and diesel?
    – TimR
    Commented May 17 at 17:21
  • @EdwinAshworth - fully agree. What my question is about is that in your second example, in some situations, someone might say "Does it run on gasoline?", using "does" instead of "could". In most scenarios this would mislead the hearer into thinking that the questioner doesn't know that it's a diesel, or they don't know that diesels use a different fuel. But if the hearer knows or assumes that the questioner knows that diesels use diesel fuel, they seem to get it and interpret "does" as "could", at least in my experience when I have observed this.
    – Steve Pemberton
    Commented May 17 at 17:22

2 Answers 2

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"Does" and "could" have different meanings. "Could" is conditional and "does" is not.

"Do" is an auxiliary verb used in questions and negations in the simple present tense. So "Does this car run on gasoline" is just the question form of "This car runs on gasoline". The simple present talks about current circumstances.

"Could" is used for the conditional, asking about different conditions - things that might happen if circumstances were different. "Could this car run on gasoline?" is the question form of "This car could run on gasoline.", a statement about something that might happen if something changed.

For example:

Does this car run on gasoline?

No. It runs on diesel.

If the car runs on both gasoline and diesel then the answer can be:

Yes. It also runs on diesel.

However if the car ran on diesel but could run on gasoline under different circumstances:

Could this car run on gasoline?

Yes, if you modified the engine.

Or another example:

Does John eat meat?

No, he's a vegetarian.

Could John eat meat?

Yes, but he prefers not to. He's a vegetarian.

Also:

Could John eat meat?

No, he's allergic and would die if you gave him meat.

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  • The conditional aspect lines up with the fact that the questioner is probably assuming that even if gasoline could be used it would be less desirable, since they are aware that diesel fuel is the primary fuel. So essentially conditional. To make your example fit better the asker knows that John is vegetarian, but they don't know if he is vegan, since some vegetarians will eat meat if that's what is served in a social gathering. So again conditional. If already established that John is vegetarian, "Does he eat meat" would probably be understood as "could" even though it's semantically incorrect
    – Steve Pemberton
    Commented May 17 at 12:34
  • I think you are making a lot of assumptions. "Does this car run on gasoline?" can be asked even if the question knows nothing about the car. Same with "Does John eat meat?" Commented May 17 at 13:02
  • DJClayworth - In most situations where these sentences are used they don't know the current situation. I am not referring to those scenarios. As described in my question the person asking the question knows something about the car, they know that it is a diesel. In my modification to the vegetarian example I said that the questioner already knows that John is vegetarian. Yet they are using "does" in the sentence even though "could" would normally be the semantically correct word. But somehow in these situations it works because the hearer knows that the questioner already knows these facts.
    – Steve Pemberton
    Commented May 17 at 13:57
  • I've edited to make this clearer. "Does" is simple present. "Could" is conditional. Note that if the car is able to run on both gasoline and diesel then "Does this car run on gasoline?" can be answered "yes"; and the same for diesel. Commented May 17 at 16:21
  • DJClayworth - agree. What I am reporting is that I have observed more than once that in certain situations which are conditional, and where the asker knows that the situation is conditional, they sometimes use the word "does" instead of the conditional "could". And on top of that the hearers seem to get it and know to essentially interpret the question as if the word "could" was used. My suspicion is that even though this seems to work it is not correct semantics, that's what my question is. Perhaps it's similar to "I could care less" which everyone seems to get even if the semantics are wrong
    – Steve Pemberton
    Commented May 17 at 18:18
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I'd say this is is strictly speaking grammatically acceptable (based on my native-speaker American-english intuition) but not desirable because of the ambiguity you identified. One might say that it is acceptable syntax but violates the cooperative principle.

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  • In some contexts it would certainly violate the cooperative principle. However in the latter example, which is what my question is really about, the hearer would likely have no problem understanding what was being asked since they know that the questioner is aware that diesel cars normally use diesel fuel. I suppose I could have gone into more detail about the preceding conversation, but I was hoping that just saying that it has already been established in the conversation that diesels use diesel fuel was sufficient.
    – Steve Pemberton
    Commented May 16 at 17:38

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