Grammar JLPT Level N3 - Lesson 16

Structure of 訳(わけ)がない:

Verb［る］⁽¹⁾ + わけがない
［な］Adjective + な + わけがない
［い］Adjective + わけがない

⁽¹⁾ Verb［た］

Details about 訳(わけ)がない:

One of the strongest ways to use the noun わけ in Japanese, is in the expression わけがない. This is regularly translated as 'there is no way that (A)', or 'it is impossible that (A)'. However, as mentioned in other わけ lessons, わけ is used very similarly to 'so' in English, so may also be thought of as 'it is SO not (A)', with 'so' being used for emphasis. In this structure, わけ is combined with the case marking particle が, and the い-Adjective ない.
わけがない may be used with verbs, or adjectives in their attributive form.

このステーキ2キロもあるの？！こんなの一人(ひとり)で食(た)べれるわけがない！
This steak weighs 2kg?! I SO can't eat something like this alone!
キムラシェフが作(つ)ったパスタだよ、不味(まず)い訳(わけ)がないじゃん！
This pasta is made by Chef Kimura, there is no way this would taste bad!
このアパートは線路(せんろ)の隣(となり)にあるから静(しず)かなわけがない。
This apartment is next to the train track, so there is no way it is quiet.

訳(わけ)がない

Usage Notes for 訳(わけ)がない:

Compared to わけではない, わけがない is much stronger, and emphasizes the 'non-existence' of something. This can be thought of as similar to the difference between 'it is so that it isn't (A)' (for わけではない), and 'it is SO not (A)' (for わけがない).

彼(かれ)はいつも遊(あそ)んでいるが、暇(ひま)なわけではない。
He is always going out, but it is not necessarily so that he has a lot of free time.
彼(かれ)はいつも仕事(しごと)をしているから、暇(ひま)なわけがない。
Since he is always working, he SO does not have any free time.

'It is so that it isn't (A)' highlights that (A) is a thing that exists, but not in the way of (A). On the other hand, 'it is SO not (A)' highlights that no part of (A) whatsoever can be considered to be correct/accurate.