이 means "this" on it's own. 것 means "thing" Putting them together (이것) means "this thing". If you just want to say "this apple", it's 이 사과. But if you want to say "this [thing] is an apple", it's 이것은 사과예요 or 이것은 사과입니다

이 goes in front of nouns, 이것 means just 'this'