I'm not in the office today.

not Adverb

— "Not" makes the verb of a sentence have the opposite meaning. I was not there. I am not fat!

not Conjunction

— The item before "not" is more correct or better than the item after "not". I wanted tea, not coffee! Meaning: I wanted tea. I did not want coffee. It's stupid, not funny. Meaning: It is stupid. It is not funny.

not Interjection

— Used to indicate the sentence before is sarcastic or ironic. This means that the sentence has the opposite meaning. I like doing lots of boring homework. Not! Meaning: I do not like doing lots of boring homework.

in Preposition

— Used to show that something is inside something else. The cat is in the box. — Used to show that someone is at home, or is available. Is John in? The Doctor is now in. — Used to show movement towards the inside. The rain came in through the window.

the Determiner

— Used, instead of a, to reference something specific, already known to exist. Compare "I read a book." and "I read the book." — Used with a stress, to show that the word following is special. Are you the John Smith that I went to school with? — Used with an adjective that acts like a noun to mean all of the people concerned The poor are always with us. — Used with superlatives forms of adjectives and adverbs. You are the best.

office Noun

— An office is a room or building where people sit at desks and work. Her office is in the third door on the left side of the hall. We're meeting at the office after work. — A position held by public election is an office. They both decided to run for office. He holds the office of senator.

today Pronoun

— Today is the current day, the day that is happening now, the day between yesterday and tomorrow. Today is the first day of the rest of your life. I have class today. Today's meeting should be short. — Today is modern times; the present in general. Today we don't have many of the old problems but we have many new ones.