word choice - Modal usage: would, could

In this sentence would it be better to use 'would' or 'could?' What's the difference?

I [would/could] go to the party, but I wasn't invited.

Answer

There are many different uses of would and could, and depending on the context, using could may or may not make sense here.

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I think would here would take the sense of

would (willing) - used to talk about what someone was willing to do or what something was able to do

Thus, to say

I would go to the party, but I wasn't invited.

expresses that you would like to go to the party— but will not because you were not invited.

But you could use could here in the sense of

could (possibility) - used to express possibility, especially slight or uncertain possibility

Suppose, for example, I am weighing my options for what to do on the weekend.

We could go on a picnic, but it might rain. I could go hiking, but my knee might act up again. I could go to the party, but I wasn't invited.

You acknowledge the possibility of those activities, but you do not necessarily desire to do so, and think it unlikely you will, because you anticipate some problems.