grammar - What's the difference between 「そうもない」 and 「ようがな い」

I know that both of these two grammars have the meaning of "it's impossible to do sth" but in the following 2 sentences, only ようがない is the correct answer.

①一方的に責められたら、こちらも説明（ a/しそうもない b/しようがない )

②ここまで話が進んでしまったらもう断り（ a/そうにない b/ようがない )。こうなったら引き受けるしかない。

Why cannot そうもない・そうにない be used?

How can I differentiate between the usage of these two grammar points?

Answer

「～～そうもない」 and 「～～ようがない」 are vastly different from each other in meaning and usage, and there is no interchangeability between the two.

「Verb in [連用形]{れんようけい} + そうもない」

= "to not look like (action described by the verb) is happening (any time soon)"

Describes "no-indication" situations.

whereas

「Verb in 連用形 + ようがない」

= "There is no way (someone) could perform (action described by the verb) even if (someone) wanted to"

Describes "no-choice" situations.

In the two example sentences,

1)「一方的に責められたら、こちらも説明（　　　）」

Only 「しようがない」 fits in. That is because the speaker would like to explain if it were all possible, but it was made impossible by the other person blaming the speaker one-sidedly.

This is a no-choice situation.

Another way to look at it is that 「こちら」 means "I/we" in this context. You would not normally use 「そうもない」 to talk about your own (possible) action.

2)「ここまで話が進んでしまったらもう断り（　　　）。こうなったら引き受けるしかない。」

Only 「ようがない」 can fill the blank. Hints are all over the context, really. 「ここまで話が進んでしまったら」 is one, and 「引き受けるしかない = "to have no other choice but to take it"」 is another. If "to take it" is the only choice you have, there is no way you can 断る ("to refuse"), is there?

Another no-choice situation, totally.