必ず

1. Origins, structure, and basic meaning

• Structure: 必ず (かならず) is an adverb meaning without fail or certainly.
• Basic usage: It emphasizes certainty or firm obligation in habits, rules, and promises.
  • 外出するときは必ず窓を閉める。 — I always close the windows when I go out.
  • 約束した時間までに必ず戻ります。 — I will return by the promised time without fail.

2. Common pitfalls and similar expressions

• Frequency and certainty
  • いつも describes something occurring habitually, while 必ず emphasizes that an action or result has no exception or that a commitment will be fulfilled.