てしまいました vs ちゃいました

These forms share the same core meaning (completion, often with regret), but tone differs.

Quick difference

• てしまいました: formal or neutral-polite
• ちゃいました: casual contracted form

Typical usage

• 宿題を忘れてしまいました。

  • "I forgot my homework." (polite)

• 宿題忘れちゃいました。

  • "I forgot my homework." (casual)

Nuance

Both can carry "unfortunately" or "by mistake" depending on context.

Rule of thumb

If you are speaking politely (customer situations, workplace, strangers), use てしまいました. With friends and casual chat, ちゃいました is natural.