てしまいました vs ちゃいました

Both forms express regret, completion, or unintended actions, but they differ in formality.

てしまいました is the polite past form - use this with strangers, at work, or in formal situations:

ちゃいました is the casual-polite form - it's friendly but still shows politeness. Common among friends who use です/ます:

When to use which:

• てしまいました - Business settings, talking to seniors, formal apologies

• ちゃいました - Casual conversations with acquaintances, friendly service staff

• ちゃった - Close friends, family (fully casual)

Note: Some speakers consider ちゃいました too casual for truly formal situations. When in doubt, stick with てしまいました.