I saw the milk spill. I am a teacher. Brian is an Irishman. Seiko is a practicing Buddhist. The signals that the noun is definite, that it refers to a particular member of a group.
Coffee is a popular drink.
Japanese was his native language. Intelligence is difficult to quantify. The Japanese he speaks is often heard in the countryside. The intelligence of animals is variable but undeniable. First vs. Subsequent Mention A or an is used to introduce a noun when it is mentioned for the first time in a piece of writing.