だけ

1. Origins, structure, and basic meaning

• Structure: Add だけ after a noun or quantity to limit its scope.
• Basic usage: 水だけ飲みました。 limits what was drunk to water.

2. Common pitfalls and similar expressions

• Role being limited
  • 私だけが読む limits who reads, while この本だけを読む limits the book being read. A required particle can follow だけ.
• しか and negation
  • Both 水だけある and 水しかない can describe a situation with nothing but water. しか combines with negation, but だけ itself does not require a negative predicate.