A inquiry regarding the usaged of preposition

Following question is the grammatical point that I want to make clear. Also, I would like to ask for writing corrections to every sentence I wrote here. <in vs. at> We say, “I was at the English Department.” but not, “I was in the English Department.” Why is that so? How can we know which one is right for different situation or sentences? Could you give me some examples with explanation?

[이름], hope this helps you using prepositions in and at talk to you soon. Hope you have a great  weekend!! Following question is the grammatical point that I want to make clear. Also, I would like to ask for writing corrections to every sentence I wrote here >>The following questions is the grammatical point that I want to make clear. Also I would like to ask for writing corrections for every sentence I have written.     We say, " I was at the English Department," or should it be "I was in the English Department." Which is correct, how can we know which one is right for different situations or sentences? Could you give me some examples with explanations   preposition in 1. (used to indicate inclusion within space, a place, or limits): walking in the park. 2. (used to indicate inclusion within something abstract or immaterial): in politics; in the autumn. 3. (used to indicate inclusion within or occurrence during a period or limit of time): in ancient times; a task done in ten minutes. 4. (used to indicate limitation or qualification, as of situation, condition, relation, manner, action, etc.): to speak in a whisper; to be similar in appearance. 5. (used to indicate means): sketched in ink; spoken in french. 6. (used to indicate motion or direction from outside to a point within) into: Let's go in the house. 7. (used to indicate transition from one state to another): to break in half. 8. (used to indicate object or purpose): speaking in honor of the event     preposition at   1. (used to indicate a point or place occupied in space); in, on, or near: to stand at the door; at the bottom of the barrel. 2. (used to indicate a location or position, as in time, on a scale, or in order): at zero; at noon; at age 65; at the end; at the lowest point. 3. (used to indicate presence or location): at home; at hand. 4. (used to indicate amount, degree, or rate): at great speed; at high altitudes. 5. (used to indicate a direction, goal, or objective); toward: Aim at the mark. Look at that. 6. (used to indicate occupation or involvement): at work; at play. 7. (used to indicate a state or condition): at ease; at peace. 8. (used to indicate a cause or source): She was annoyed at his stupidity.