"Is there a thin pen?"
When do you use ほそい and うすい? As well as あつい and ふとい? Can't they be used interchangeably? I used うすい and it's wrong.
Apparently, 細い (ほそい）is for thin people and cylindrical objects while 薄い (うすい) is for thin, flat objects. Similarly, 太い (ふとい）is for people and round, cylindrical objects and 厚い (あつい) is for flatter objects like books.
If I were to say a skirt is ほそい, I would be saying it had a slender shape (thin) If I were to say a skirt is うすい, I would be saying it the cloth it was made from was lightweight (thin)
ほそい is used for shape: slender, thin, fine; and to mean there is little of something (e.g. food, volume, etc.)
うすい is used for thinness in the sense of substance, so is used to describe things like soup (meaning watery), color (meaning pale), taste (meaning flavorless), probability (meaning low), etc
Hopefully that doesn't make it more confusing :/
I seem to remember somewhere that は took precedence over が . So as this is the subject it gets は and the が is .. kind of ...hidden.