This pattern is used to say that something is required or obligatory. It is formed by taking the negative form of a verb or adjective and replacing the final ない with なければいけない. For verbs, the negative stem is the same as the ない-form without the ない, e.g., 食べない (tabenai) -> 食べなければいけない (tabenakereba ikenai). For nouns and な-adjectives, use でなければいけない, e.g., 学生でなければいけない (gakusei de nakereba ikenai) meaning 'must be a student'.

This pattern is common in everyday conversation and is acceptable in both spoken and written Japanese. However, it can sound a bit strong or direct, so in polite conversation, you might soften it with んです or use the more polite form なければなりません. But なければいけない is perfectly fine for stating rules or personal obligations.

Remember that いけない literally means 'not good' or 'cannot go', so the pattern roughly translates to 'it is not good if not (do)'. This helps understand the nuance: it is not an option, but a necessity.