A determiner is a word that goes before a noun and describes how we are referring to it in the sentence. Determiners can be compared to adjectives, which express how something is. Determiners can include demonstratives such as that, articles such as the, and possessive pronouns such as his.
This is the third drink you've had.
In the classroom
Realia is a good way to demonstrate determiners such as ‘this' and ‘that', for example by asking learners to play games such as I-spy.