当然

1. Origins, structure, and basic meaning

• Structure: 当然 is read as とうぜん and functions as a noun or adjectival noun for a result judged natural, reasonable, or only to be expected.
• Basic usage: A general obligation appears in 返すのは当然だ, while an expected result appears in 勝つのも当然だ.

2. Common pitfalls and similar expressions

• Difference from 必然
  • 必然 is logical inevitability; 当然 is a judgment that something follows reasonably from norms or circumstances.
• Speaker evaluation
  • The word can convey the speaker’s assessment that a fact is natural or justified, not only the bare fact itself.