Using This/That with 이다
Remember, 이다 translates to "to be" and is conjugated as "am/is/are" in English. Now that we know how to use 이, 그 and 저 (and 이것, 그것 and 저것), we can now make sentences like this:
That person is a doctor
We can start by putting those words into the Korean structure:
That person는 doctor is
And then changing the English words to the appropriate Korean words:
그 사람은 + 의사 + 이다
그 사람은 의사이다
(그 사람은 의사야 / 그 사람은 의사예요)
More examples:
그 사람은 선생님이다 = That person is a teacher
(그 사람은 선생님이야 / 그 사람은 선생님이에요)
이것은 탁자이다 = This (thing) is a table
(이것은 탁자야 / 이것은 탁자예요)
저것은 침대이다 = That (thing) is a bed
(저것은 침대야 / 저것은 침대예요)
그 사람은 남자이다 = That person is a man
(그 사람은 남자야 / 그 사람은 남자예요)
그 사람은 여자이다 = That person is a woman
(그 사람은 여자야 / 그 사람은 여자예요)
그것은 차이다 = That (thing) is a car
(그것은 차야 / 그것은 차예요)
이것은 나무이다 = This (thing) is a tree
(이것은 나무야 / 이것은 나무예요)
Wow! That was an extremely difficult lesson. If you were to pick up another Korean text book, I am sure the first chapter would be much easier than this. Trust me though; learning this at the start will be very useful to you later on. When I was learning how to speak Korean, it took me months to realize some of these things (not because they were hard, but because I was using a text book that never taught me the reason why things are the way they are in Korean).
Before you move on, make sure you understand the simple Korean sentence structure presented in this first lesson. Also, remember that the sentences not in parentheses are technically incorrect (or very very uncommon) because they have not been conjugated.