I can't go: 行けない? 行けそうにない？

Medium-level students of Japanese will certainly be familiar with the construction なになにしそうにない (nani-nani shisō-ni nai), meaning something like “It doesn’t look like I’ll be doing X”. But what exactly is the difference in nuance between this and simply saying なにないしない (nani-nani shinai), meaning “I won’t be doing X”? When is this そうにない (sō-ni-nai) form called for? Is it stronger, or weaker than the more basic form, or does it have some additional nuance?

If you’re asked whether you’re going to a party, say, you might answer 行けない (ikenai), or “I can’t go”, but you might also respond 行けそうにない (ikesō-ni nai), or ”It doesn’t look like I can go”. Again, what’s the difference?

According to one trusty native informant:

I think they mean the same thing, with maybe the first one being slightly more unconditional.

I feel like the main difference is the first one is more terse and direct.

Another informant provides us with more detail:

I also think it’s “行けない” that is absolute and final. 

But if I imagine my friends or coworkers say “行けない”, then I might ask why not. And if they say “行けそうにない” then I would also take it as no but probably wouldn’t ask why.

It’s probably because “行けそうにない” hints that the person has an intention to go but there is a factor outside his or her control that makes it difficult for him to go. So it feels rude to keep asking questions or pressuring him to go.

There you have it. To simply state the absence of something—like your ability to go—use the simple negative, like 行けない (ikenai), “I’m can’t go”. To soften your refusal to go a bit, and imply some possible factors at work, use 行けそうにない (ikesō-ni nai), “I don’t think I’m going to be able to go”.

It turns out that Bob has been under the misimpression for many years now that the form そうにない (sō-ni nai), or “doesn’t look like”, was the stronger, more categorical, more unconditional form, the nuance being “There’s simply no way I’m going to be able to go”. That kind of thing can happen when you learn Japanese totally in context, like in bars in my case. Now I know.