Contrastive は

A nuance of は is that it can be used for contrasting two things. This is sometimes referred to as “comparative” は。 This page will explain the basic pattern, and some other usages of it including ~くはない and なくはない.

Double は

AはXがBはY

The stereotypical way は is used to show contast is with 2 は like this:

AはXがBはY

X and Y may be the same thing, if a word like もっと (more) is used.

Examples

• 風は強いが、日は照っている (Weblio)
  The wind is strong, but the sun is shining. (So its not stormy)
  
  
• 彼は頼りにならないが、彼女はしっかりしている
  He’s unreliable, but she’s got it together.
  
  
• 海は広いが、船は小さい.
  The sea is vast, but a ship is small. 
  
  
• 近所はいいが、家は狭い
  The neighborhood is good, but the house is small.
  
  
• 彼女は賢いが、先生はもっと賢い
  She’s clever, but the teacher is more clever. 
  
  

Single は

This contrasting meaning of は carries over to when its being used by itself as well.
The nuance is always there in the background, but is more pronounced in some situations.
Most typically, if は replaces a が that should be there like が好き、がある、ができる, etc;
Then most likely you have a comparative は nuance. 

Examples

• 猫は好き
  I like cats… Probably something omitted like:
  猫は好きけど、犬はもっと好きだ
  I like cats, but I like dogs more. 
  
  
• 考えはある
  I have an idea (But maybe not a plan)
  
  
• 本はよめる.
  I can read a book. (but maybe not a newspaper).
  
  
• 書くことはできる
  I can write. (But maybe not read?)
  
  

～くはない

Not that “Adjective”
い Adjectives: くない→くはない

Contrastive は loves to pop up in い Adjectives in particular.
It comes before the ない part, transforming it into くはない.
Basically, it “hedges” the statement a little, kind of like adding an implied “but” to the statement.

くはない can also be くもない.
When listing both out, the second くはない frequently becomes くもない

＊Note: Annoyingly, there is another もない that means “not even”. 

• 悲しくもないのに、なんで泣いている
  You’re not even sad, why are you crying. 
  
  
• 1つもない
  Not even one.
  
  
• 考えたこともない
  Never even thought about it. 
  
  

Examples

• 彼は優しくはないが、（厳しくもない）
  He’s not nice but he’s not strict either.
  
  
• 今の仕事は悪くはない
  This job isn’t bad, (But not really good either).
  
  
• 外は寒くはない
  Its not cold outside (but not warm either).
  
  
• 嬉しくはないが、大丈夫
  I’m not happy (I’m something else), but its ok. 
  
  
• 嬉しくはないが、悲しくもない
  Not happy, But not unhappy.
  
  

なくない and なくはない

Not “Not X”/It’s not that I don’t…
Any ない -> なくない

なくない is probably the hardest double negative to understand at first.
It took me a while to get it too, so don’t worry if you have trouble.
It is a negative (ない) turned into an adverb (なく) modifying another negative (ない).

Its a roundabout way of saying something is something, but minimizing the degree.
It is a way of hedging a statement. 
If it helps, think of this pattern as ないことはない (Basically identical in meaning).

なくない can also be used with comparative は as なくはない. It may also be なくもない
Meaning wise, it really doesn’t change that much though.

Examples

なくない

• 優しくなくない
  Not “Not nice”
  
  
• 暇じゃなくない
  Not “Not free”
  
  
• 食べたくなくない
  Not “Not want to eat”  I don’t not want to eat it. 

なくはない

• 食べたくなくはない
  I don’t not want to eat it, but…(I kind of want to eat it but..)
  
  
• 飲みたくなくはないが。。
  Its not that I don’t want to drink, but (I don’t want to be hung over). 
  
  
• 食べなくはないが、あまり食べない
  Its not that I don’t eat (it), I just don’t eat it a lot.
  
  
• 勉強しなくはないが、
  Its not that I don’t study but..(I don’t study much, etc).

Example Situation

• 飲みたくない？
  You don’t want to drink?
  飲みたくなくはないが。。
  I don’t not want to drink but..
  
  
• 寿司を食べることある？
  You eat sushi?
  食べなくもないが。。
  Its not that I don’t eat sushi but..
  
  
• 映画を見に行くことある？
  Do you go to see movies?
  見に行かなくないが。。
  Its not that I don’t’ go see them.