はしない・
はしない and やしない strongly deny that an action happens or will happen, usually in response to an expectation, claim, fear, or proposed possibility. The second form is distinctly more colloquial and forceful.
What the form pushes back against
This is not simply a longer version of an ordinary negative. It presents the action as a possibility already under discussion and firmly rejects it: “That will not happen,” “I certainly will not do that,” or “It is not true that someone did that.” The earlier possibility may have been stated aloud, implied by the situation, or feared by the listener.
Rejecting an expectation
また途中で諦めるんでしょう。
You are going to give up halfway again, aren't you?
Analyze諦めはしない。今度は最後までやる。
I will not give up. This time, I will see it through.
AnalyzeThe first speaker supplies the expectation; the second firmly rejects it.
The difference in tone
はしない can sound controlled and deliberate. やしない often sounds more emotional or conversational.
Past and polite forms
When the denied event belongs to the past, change the ending to the past negative. In polite speech, the strong denial can remain, but the standard form is normally the better fit.
Not doing something versus not even doing it
はしない and やしない broadly reject the possibility that an action occurs. By contrast, the form with も often means that someone fails to do even the minimum expected action. Choose according to what you are emphasizing: denial of the event itself, or omission of even a basic step.
More examples
彼が金で動くと思っているのか。そんな条件では動きやしないよ。
Do you think money will sway him? He would never budge on those terms.
Analyze心配しなくていい。君一人に責任を負わせはしない。
You do not need to worry. I will not make you bear the responsibility alone.
Analyze全員が賛成したと言うが、私は賛成しはしなかった。
They say everyone agreed, but I did not.
Analyze彼が約束を忘れたって？忘れやしないさ。
You think he forgot his promise? He would never forget it.
Analyze期限を延ばすべきだという意見もありますが、方針を変えはしません。
Some have argued that the deadline should be extended, but we will not change the policy.
Analyze逃げたという噂だったが、彼は町を離れはしなかった。
Rumor had it that he fled, but he did not leave the town.
Analyze