Hi Anastasia, To answer your question: YES because it can be substituted, and it depends on the use of the word. Articles ( indefinite “a”, “an” / definite“the”) can be substituted depending on what you wanted to say. With your first sentence: “The people that I AM closest to, I met in the choir” (substitute) or “The people that I AM closest to are the ones I met in the choir” (substitute) Another use of this word: ex: “Jane is friendly, among my friends in the choir community, she is THE closest. [OR] “Among my friends in the choir community, she is MY closest friend. (substitute) In reference to: “I am closest to my mother” there should be a context before or after this sentence. If none, allow me to add a few notes... You may say: “I am close to my mother” (as we only have 1 mother and there’s no comparison, unless you wanted to state a situation where you were not close to someone but rather your mother then you wanted to say something like this: [“I am closer to my mother, than my father”] as we only have 2 parents, so the use of "closer" is more appropriate in these sense. or you wanted to emphasise ("emphasize") something “Among my relatives I am closest to my mother” (a comparison to numerous people) Have a great day!