When you want to make the predicate (verb, adjective, adverb, auxiliary verb, etc) of the sentence negative you put ไม่ in front of it. When you want to make the subject (noun, noun clause) negative, you put ไม่ใช่ in front of it.
1. I'm not going.
ไม่ไป
2. It's not spicy.
ไม่เผ็ด
3. He/she is not American.
เขาไม่ใช่คนอเมริกา
4. This isn't a book.
นี่ไม่ใช่หนังสือ
5. I don't like watching.
ไม่ชอบดู
