这 / 那【zhè / nà】

这 points at what is near the speaker, 那 at what is further away. Alone they work as the subject of a sentence.

In front of a noun, a measure word is required

zhè这Shū书 is not how it is said. 这 and 那 need a measure word before the noun, the same one the noun takes when it is counted.

这个 and 那个 are by far the commonest, because 个 is the default measure word. Anything with its own measure word keeps it: 这杯茶, 那张照片.

Plurals

这些 and 那些 cover "these" and "those", and they take no measure word.

这 and 那 in speech

Both have a second pronunciation, zhèi and nèi, heard constantly in Beijing speech and written with the same characters. 这个 as zhèige is not a different word.

那 also means "in that case"

At the start of a sentence, 那 stops pointing at anything and links to what was just said.