[Grammar] differences between can and could

[macellan]

#1
[FONT=&quot]What do you want to do? - Well, we ___ have a picnic, but it looks like rain.
I’m so hungry I ___ eat a horse!

Why can we not use "can" instead of "could" ? Both of them point out possibility.

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[jutfrank]

The idea of possibility is rather complicated and abstract, so I'll try to answer your question simply and practically.

It will be useful for you to think of these sentences in terms of their functions.

Your first sentence is a suggestion (a suggested possibility). To make this kind suggestion, use We could ...

Your second sentence is a statement of potential ability (what it is hypothetically possible for you to do), so I could ... is the right way to express this.

 

[Glizdka]

Not a teacher
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I like to think of "could" as a combination of "can" and "would" (can + would = could). If your sentence with "can" could use a bit of wouldness in it (to make it hypothetical or more polite), you should use "could". But then, "could" is used in many other, different ways too, i.e. it's sometimes referred to as "the past version of can".

 

[Yankee]

#1
What do you want to do? - Well, we ___ have a picnic, but it looks like rain.
I’m so hungry I ___ eat a horse!

Why can we not use "can" instead of "could" ? Both of them point out possibility.

I consider "can"(ability) and "could" (possibility) in their simplest meaning. "We can have a picnic. I have all the supplies". "We could have a picnic, but it looks like rain."