Can vs. Could in the reply "It can/could be in the car"

Question

I’ve made another mistake in a modals test, in the can/could segment. The following exchange was presented:

Where’s my bag? Have you seen it? – No, but it _ be in the car.

I decided that both ‘can’ and ‘could’ are appropriate in the underscored position. The key contained only ‘could’. I’m curious why ‘can’ here is wrong:

Where’s my bag? Have you seen it? – No, but it *can be in the car.

In the Russian language, we use only the present-tense form of ‘can’ in such hypothetical answers: the past of 'can' would sound literal and ridiculous; so one may tend to carbon-copy the 'can' construction into English, failing to notice the difference in sense.

I’ve consulted a textbook and it says that ‘can’ tends to be used to convey a more general meaning. An example is given:

The weather can change very quickly in the mountains. (in general)

The weather is nice now, but it could change. (the weather now, not in general).

Is it in the same vein with the car in the example? Does “It can be in the car” sound too general while “It could be in the car” zeroes in on the immediate situation?

Would the sentence with ‘can’ come out too haughty in the context, that is, a person turns for advice concerning situation at hand, and gets a generic sentence, almost telling him “well, your bag, speaking generally, can fit in the car, go guess the rest of it”?

Answer 1

Modal verbs can be a little tricky. When you suggest a possibility for a specific situation at hand, such as "Where is my bag (right now)?" you are hinting at where you think it "might" be. Since you are only talking about the bag in this specific instance you should use "could" or "might" to indicate you are responding to this situation. As you also correctly discovered, "can" is reserved for "general truths" (a kind of zero conditional: "If you smoke too much it can kill you").

The examples your textbook gave you are good. Try to contrast "right now/this specific situation" = "could" vs. "in general" = "can", as a rule of thumb.

To answer the last part of your question, "can" just doesn't really fit in this situation. Since you were asked about a specific situation "Where's my bag?" you really can't use "can". You can be plenty haughty by choosing your phrasing rather than your modal:

No, but it could be in the car along with your head that you clearly left there!

Lastly, and just a piece of advice, from the way I view "can" and "could" as possibility modals: Don't think of them as present and past (like when they describe ability/permission), but rather: general = can, specific = could.

Answer 2

Can is not often used to express possibility. I think in British English it is rare or non-existent to do so, and in American English we don't usually use can with this meaning. Instead, could is used, or may/might.

So to express

It is possible that the bag is in the car.

use

It may be in the car.
It might be in the car.
It could be in the car.

Can is barely, if at all, workable for me (AmE) with this meaning. The last two, might and could, express a lesser degree of possibility that the bag is in the car.

To express the opposite

It is not possible that the bag is in the car.

We do use can't/cannot.

It can't be in the car. I took it out of the car and put it somewhere else.

We don't use may not, might not to express this negation.

There is another negation, the one that states

It is possible that the bag is not in the car. I am not sure if it is there or not.

Here, we use It may/might not be in the car but not can't/cannot or couldn't/could not for this. I know few people who would say It mightn't be in the car and no one who would say It mayn't be in the car.

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The sentence

The weather is nice now, but it could change.

is about possiiblity. This is the same as the sentence about the bag.

Again, to express

It is possible that the weather change

the use of could is much more idiomatic. As is may/might.

The other sentence

The weather can change very quickly in the mountains.

is not about possibilty, but ability. Note that we can be sure about this by substituting could. I've said above that could is idiomatic for possibility; but could here

&&The weather could change very quickly in the mountains.

is not at all felicitous to express possibility. This is in contrast to the felicitous use of could in the sentences above to express possibility. (Note that the sentence is grammatical to express past ability.)

Instead, can is being used in its "dynamic" sense of expressing ability. Note, of course, that 'the weather' does not have a will of its own. But we often ascribe ability to inanimate objects. Compare:

This Porsche can accelerate from 0 to 60 in 3.1 seconds.

This is talking about ability. This is different from expressing possibility. And a Porsche salesperson knows that quite well! Using could here would represent either past ability (This Porsche could accelerate from 0 to 60 in 3.1 seconds when I first bought it) or irrealis (This Porsche could accelerate from 0 to 60 in 3.1 seconds if you fitted it with a more powerful engine).

Can in the sense of ability very often is replaceable by is able to.