1. 이
meaning : this (adjective)
e.g. 이 가방은 누구 거예요? (Whose is this bag?) -polite ending form
e.g. 이분이 누구세요? (Who is this person?) - honorific form
2. 이것 (pronoun)
meaning : this one, this thing
e.g. 이것 좀 보세요! (Look at this!) - polite ending form
3. 이것은 (pronoun + topic marker '은')
e.g. 이것은 누구 거예요? (Whose is this thing?)
* topic marker 은/ 는 is used when you are talking about something as a topic.
After a final consonant of a syllable called 받침, '은' comes but after a vowel, ' 는' comes instead.
** '이것은' usually changes to '이건' in daily conversation.
e.g. 이건 뭐예요? (What is this?)