No Sign of It Happening
そうにない says that, from what you can see, something is not going to happen.
- 今日中に終わりそうにない。(It doesn't look like it'll be finished today.)
It is a judgement from appearances, not from information — that is what separates it from ないでしょう.
It Is the Negative of そう
| Form | Meaning |
|---|---|
| 降りそうだ | looks like it'll rain |
| 降りそうにない | shows no sign of raining |
Note the form: it attaches to the ます stem, so 降ります → 降りそうにない. Not the dictionary form.
そうにない vs そうもない
Interchangeable. そうもない is marginally stronger, the も adding "not even".
Not the Same as なさそう
A trap worth naming:
| Pattern | Meaning |
|---|---|
| 来そうにない | doesn't look like he'll come |
| 来なさそう | looks like he isn't coming |
Both are negative, but そうにない negates the likelihood while なさそう takes the appearance of an already-negative state.