This lesson will discuss は’s role as an indicator of contrast between more than one thing.
は may highlight the scope of a sentence by indicating contrast and can do so even when there aren't two things stated. In a negative sentence it emphasizes negation. By doing so, the emphasis is still placed on what は marks. Overall, though, a negative sentence with は is markedly noticed. は can be seen after other things such as other like other particles and even adverbs like "always".
I can hear the dog, but I can't see the dog.
Grammar Note: While the first は is thematic, the second shows contrast.
I won't go. (Giving the emphasis that you aren't and don't want to.)
There is no ballpoint pen.
I always go.
I'll pay the money.
Particle Note: よ marks emphasis and puts stress on the fact that you're paying.
Japanese is hard, but English isn't hard.
I have a pen, but I do not have a pencil.
You don't know, but everyone else already knows.