くは

くは puts the contrastive は between an い-adjective in its く-form and ない: たかく高くはない, "it isn't expensive, as such". The は marks the denial as limited to that one quality, and the sentence usually goes on with a but.

The polite forms are くはないです and くはありません, and the past is くはなかった.

くない or くはない

くない is a plain no. くはない says "not X – but". It can also hedge a judgment, which makes it the usual way to say something is decent without praising it.

• × 難しいはない → 難しくはない

Usage notes

These go beyond the negative.

くはある

With ある the sentence grants the quality, then takes something back with が or けど. It belongs to writing and careful speech.

な-adjectives and nouns

A な-adjective or a noun takes では in the same place: しずか静かではない, しずか静かではある. In conversation that becomes じゃない.