くはEmphasis and Contrast

Besides marking the topic of a sentence in a straightforward way, the は particle can also be used to indicate contrast or comparison. One way it does this is with い-Adjectives, in the pattern ～くは.

Grammatically, this is the continuative form of an い-Adjective (く), plus the binding particle は.

～くは is most often used with negatives, to emphasize that something isn't necessarily one thing, but there is an implication that it still may be another. This looks similar to the regular negative form of い-Adjectives but with は inserted after く.

As は is being used contrastively here, often the contrast will be explicitly stated, although this isn't always the case.

Although this pattern of inserting は for contrast is most common with negatives, it can also be seen in positive sentences.

Fun Fact

Although the focus of this grammar point is ～くは, this contrastive use of は can be seen in many different contexts.