もしない details
write + doesn't even — marks that even a minimal, expected act didn't happen, against a backdrop of contrast
kaki mo shinai
doesn't even — marks that even a minimal, expected act didn't happen, against a backdrop of contrast
meaning: not even write
あれほど世話になったのに、お礼の手紙を書きもしない。
are hodo sewa ni natta noni, orei no tegami o kaki mo shinai.
Despite all he'd done for me, I not even write a thank-you letter. neutral
あれだけ迷惑をかけたのに、あいつは謝りもしない。
are dake meiwaku o kaketa noni, aitsu wa ayamari mo shinai.
After causing all that trouble, that guy won't even apologize. casual
- past
- polite
Examples
doesn't even — marks that even a small, expected act failed to occur, typically against an implied or stated contrast
あれほど世話になったのに、お礼の手紙を書きもしなかった。
are hodo sewa ni natta noni, orei no tegami o kakimoshinakatta.
Despite all he'd done for me, I didn't even write a thank-you letter. neutral
あれだけ迷惑をかけたのに、あいつは謝りもしない。
are dake meiwaku o kaketa noni, aitsu wa ayamari mo shinai.
After causing all that trouble, that guy won't even apologize. casual
問い合わせのメールを送っても、先方は返事を書きもしません。
toiawase no meeru o okuttemo, senpou wa henji o kaki mo shimasen.
Even when we send inquiry emails, the other party won't even write back. formal
Formation
{V-masu-stem}もしないattaches to ます-stem
Notes
Nearly always paired with an explicit or implied contrast — the expected act is named, then denied.
Related patterns
Similar
Both mark an act falling short, but ろくに〜ない says the act, if it happened at all, was done poorly or insufficiently, while Vもしない says a minimal expected act didn't happen even once — usually set against an explicit contrast (それなのに) rather than criticizing HOW it was done.
Lookalikes
Both fuse も and ない onto a verb form and look like one family, but そうもない negates an appearance-based auxiliary — "shows no sign of happening" (晴れそうもない, doesn't look like it'll clear up) — while Vもしない negates the act happening at all — "didn't even do it" (謝りもしない, didn't even apologize).