该【gāi】

该 before a noun points back to something the text has just named: "the said company", "this school". It is formal and written – news reports, official notices, product descriptions. No measure word comes between 该 and the noun.

The 该 of 应该, "should", is the same character. Formal 该 is followed by a noun; "should" 该 is followed by a verb.

该 or 本

本 is the other formal demonstrative. 本 means "our own, this one here" – the speaker's company, this very shop. 该 means "the one mentioned", usually someone else's.

Not in conversation

In speech, 该 for "this" sounds like a press release. Use 这 or 那 with a measure word.

• × 我昨天看了该书 → 我昨天看了这本书
• × 该个学校很大 → 该学校很大 / 这个学校很大