此【cǐ】

此 means "this" in formal writing and notices, where speech puts 这 or 这个. It takes no measure word: 此人, 此事.

• × 此个手机是我的 → 这个手机是我的

After a preposition

此 can follow a preposition on its own, meaning "this place", "this point" or "this matter": 在此 "here", 至此 "up to this point", 对此 "about this", 为此 "for this reason". 这 can't stand alone there; it needs 里, 个 or a noun.

• × 对这，学校还没有回应 → 对此，学校还没有回应 / 对这件事，学校还没有回应

Notices and set words

Signs and tickets use 此 for the place or item in front of you: 此路不通 "no through road", 此票 "this ticket". Many common words are built on it – 因此, 如此, 从此, 此外, 彼此 – and several have their own entries.

In conversation 此 sounds official. Saying 此人 about a friend comes across as a joke.