certainly

1. Origins, structure, and basic meaning

• Structure: 必 has 5 strokes, the on-reading ひつ, and the kun-reading かならず. 必ず (かならず) means without fail.
• Basic usage: 必要 (ひつよう) identifies something needed; 必ず describes an action done consistently or promised with certainty.

2. Common pitfalls and similar expressions

• Necessity and certainty
  • 本人確認書類が必要です means that identification documents are required. 必要な道具だけ refers only to the necessary tools.
  • 必ず窓を閉める describes always closing the windows when going out. 必ず戻ります is a firm promise to return without fail.