이것이 책상입니다. This(not that) is a desk. (comparing two things, focusing on a subject) 이것은 책상입니다. This is a desk. (introducing an object) or This is a desk, not a chair. (focusing on the thing that is being explained.) e.g 1. 냉면은 좋아? 국수는 좋아? -> 냉면이 좋아? 국수가 좋아? (comparing 냉면 with 국수, focusing on 냉면, 국수) 2. 팔이 괜찮은데, 목은 아프다. -> 팔은 괜찮은데, 목이 아프다. (focusing on 괜찮은데, focusing on 목)

They are almost same but have some little difference in nuance. Koreans usually use 은/는 more. People use those in normal. 이/가 is used in the case you reply to the question like '책상이 뭐야?(what is desk?)'. Then you can say '이것이 책상이야(this is a desk)' Generally, the best and easiest way to choose between 은/는 and 이/가 is selecting one up to the question. When people ask with 이/가, you just use them either. Except indicating people (저 사람이 수지야 she is su ji) or asking (침대가 뭐야? What is chim dae?) or some special cases, you can usually use 은/는.

When you want to say "This is a desk.", you can say 이것은 (어떤) 책상입니다. When you want to say "This is the desk.", you can say 이것이 (그) 책상입니다.

Thanks for your answers ^^ they all helped me to understand better!! 감사합니다!