此【cǐ】

此 means "this". It is the written counterpart of 这, and it sits straight before a noun, with no measure word. You'll see it on signs, in notices and in news.

The pairs you meet first

Most of the time 此 comes inside a short fixed pair. Each one has an everyday equivalent with 这.

• 此时 – 这时, "at this moment"
• 此次 – 这次, "this time"
• 此处 / 此地 – 这里, "here"
• 在此 – 在这里, "here", as in "we hereby…"
• 从此 – "from then on"

从此 marks a point in a story and says what held true from then on. Its everyday form is 从那以后.

Not in conversation

In speech, and wherever you would need a measure word, use 这.

• × 此个问题很难 → 这个问题很难
• × 我很喜欢此书 → 我很喜欢这本书

此 also sits inside 因此 "therefore", 如此 "so, like this" and 此外 "besides". Those are separate words with their own points.