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.
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.