なくてはいけない

Formed from the negative te-form (e.g. 飲(の)まなくて) + はいけない. Used for personal obligations, social rules, and situational necessity. Slightly more conversational in feel than なければならない.

Forms

• Verbs: plain negative → drop ない → add なくてはいけない (行(い)く → 行(い)かなくてはいけない)
• I-adjectives: drop い → add くなくてはいけない
• Nouns / Na-adjectives: + でなくてはいけない

Casual contractions

• なくては → なくちゃ (行(い)かなくちゃ)
• なくてはいけない → なくちゃいけない / なくちゃ

Usage

• なくてはいけない: "must," often felt as a personal or local obligation
• なくてはならない: "must," slightly more formal; describes what is generally required
• なければならない: "must," the most written-style of the group

Register

Very common in spoken Japanese, including in polite conversation. The contracted form なくちゃ is everyday casual and frequent in conversation between friends or in self-talk.