未【wèi】

未 negates an event that hasn't happened, in writing. It stands where speech uses 没(有) or 还没.

尚未, 从未, 未曾, 未能

未 often travels with a partner. 尚未 is "not yet", the written 还没. 从未 and 未曾 are "never (so far)", the written 从来没. 未能 is "was unable to", the written 没能.

No 了 after the verb

Like 没, 未 denies that the event happened, so there's no completion for 了 to mark. 过 can stay.

• × 他未参加了会议 → 他未参加会议

Not for habits or plans

未 is about what has or hasn't happened. A habit, a refusal or a future plan takes 不, in writing as in speech.

• × 他未吃肉（"he doesn't eat meat"） → 他不吃肉
• × 明天我未去 → 明天我不去

Before a noun in notices

Rules and signs use 未 + verb to describe a person or thing: 未成年人 "minors", 未经允许 "without permission".

未必 "not necessarily" has its own entry; 未来 "future" is an ordinary noun.