Hi Harrison, "The", "A" and "An" are known as determiners. The serve a main purpose of providing more specificity for nouns. The main idea is: When we are referring to a non-specific noun, we use a non-specific determine ("a" and "an") Example: There are five apples. Pass me an apple. (You do not require a specific apple. You are happy having any one of the five apples.) When we are referring to a specific noun, we use a specific determiner ("the") Example: There are five apples. Pass me the apple on top. (You require a specific apple on top of the other four apples.) In your sentence: "The increase in people’s weight can be explained by the following reasons." You are probably about to introduce and elaborate on some SPECIFIC reasons. Moreover, your reader may already know the SPECIFIC reasons and are waiting for you to elaborate further. If you are referring to some non-specific reasons, the sentence will be: "The increase in people’s weight can be explained by A reason / SOME reasons." Just as a side discussion, we introduce a noun using "a". When we mention it again subsequently in the text, we can start using "the" as the reader already knows which noun you are referring to. Example: Photosynthesis is (A) process where plants convert light energy to potential energy. (THE) process requires (A) green pigment. (THE) green pigment is known as chlorophyll. Hope this helps. Cheers, Lance