My understanding of "could"

Question

1. I could play with my dear friends tomorrow at the seaside. Unfortunately, my sister happened to fall ill. I'll have to break my promise and stay at home to take care of her.

2. I could play with my dear friends tomorrow at the seaside, but this all depends on whether my mom will allow me to.

I made up the two examples. Though it's not perfect, I just use them as tokens for my understanding. The idea around them is the key point here.

From my understanding, these two could are quite different:

Could in #1 is equivalent to "would be able to", meaning that I'm not likely to go to the seaside tomorrow. But I would be able to if my sister hadn't been ill. As PEU 124.7 indicates, could is used to express irrealis ideas.

Could in #2 is equivalent to "might", meaning that I'll probably go to the seaside if my mom allows me to. This is an example of the tentative usage of could.

PEU¹ 124.7 Could can be used to criticise people for not doing things.

You could ask before you borrow my car. (irrealis usage of could)

Could have + past participle is used for criticisms about the past.

You could have told me you were getting married.

Is my understanding right?

^{1. PEU = Michael Swan's, Practical English Usage.}

Answer 1

Firstly, I'm not sure if the second clause in your first sentence is grammatical - you can't use unfortunately, but since but works as a conjunction between clauses, and unfortunately interrupts this.

Also: that first sentence makes more sense in the past tense:

I could have played with my friends, but ...

The only way I can imagine it making sense in the present tense is if it were being spoken aloud to oneself, as a train of thought.

• could expresses ability, permissibility, or possibility.
• may/might expresses possibility or permissibility.

In that regard, may can usually be replaced by could.

However, specific instances of could are defined by their co-text - in your case, what you've said is correct.

The first expresses (in)ability (though it's possibly also denoting permissibility or possibility, depending on whether you would consider it permissible or possible to go to the beach when your sister is ill, and whether or not you should be taking care of her).

It's not really criticism, since the the reason that "you/he could X" works is that it plays on a series of assumptions, for example:

"You could ask before borrowing my toothbrush!"

1. I'm telling you that you could ask, which is an inane and fairly pointless observation on its own.
2. Therefore, the assumption is that you don't ask, and
3. I want you to ask.
4. In this way, I'm criticising you for not asking.

The second expresses permissibility (but again, you could argue that it also expresses possibility, depending on whether you consider it a possibility that you would disobey your mother).