那

Before a noun, 那 almost always takes a measure word in between. The pattern is 那 + classifier + noun, as in 那个 for a person and 那 + 本 for a book. At the start of a sentence 那 alone often means then / in that case, linking back to what was just said.

In casual northern speech, especially Beijing, 那 is often pronounced nèi before a classifier - a fused form of 那 + 一. So 那个 becomes nèige, and 那 + 本 becomes nèi běn. Writing stays the same. As a rare surname 那 is first tone; in classical poetry 那 can stand in for 哪 (how / where), as in the Song line 问渠那得清如许. An archaic literary reading 那 also exists ('many; beautiful; how'; old variant of 挪 'to move'), but it's effectively dead in modern usage; you'll only meet it in classical texts.