Hello, When someone says そこに, I am forcusing on specific place to be written. For example, if i was asked そこに書いてください, I'd try to find what part of something i should write in. Whereas そこで generally means the phisycal place where i can do something. For example, if i was asked そこで書いてください, I'd think if there is appropriate place like chairs or sofa where i can sit down and write.

- 「に」: target place of action, where you do to, - 「で」＝「に」＋「て」: platform of action, where you do - 「そこ」： 1). "THERE". Usually indicating a place, closer to the listner and far from the speaker. 　　　「そこに本がある」 2). "THEN". Indicating a point of time.　 　「合図の笛が鳴ったら、そこでスタートです」 *** A: 「そこにその単語を書きます」 A-1). you should write the word there (somewhere on the paper). ※　そこ：meaning (1). *** B:「そこでその単語を書きます」 B-1: you should write the word there (somewhere in the room) ※ そこ: meaning (1) or B-2: you should write the word then. ※ そこ: meaning (2) I believe B-2 is more likely to happen than B-1.