适用于【shìyòng yú】

适用于 states the scope of a rule, a law, a method or a product: who or what it covers. 适用 is "be applicable", and 于 is the written "to", so the people or cases come straight after it.

The negative puts 不 in front. Notices often state both the scope and the exceptions.

With 的, the phrase becomes a modifier: 适用于儿童的药 "medicine suitable for children".

适用 or 适合

适合 says something suits a person – it fits their taste, size or ability, and it takes its object directly. 适用 says a rule or tool covers a case. Clothes, colours and jobs take 适合.

• × 这件衣服适用于你。 → 这件衣服很适合你。
• × 这个规定适合于所有学生。 → 这个规定适用于所有学生。

Keep 于

In a full sentence the scope needs 于. 在 is the spoken word for location and doesn't replace it.

• × 本法适用在全国。 → 本法适用于全国。