exist 通常指可观察到的或众所周知的存在现象。
be 指某时某地确存在的事物或现象，多与there连用。
live 多指有生命的东西的存在。指无生命事物时，强调可联想起生命的那些特点。
参考例句
1. “To be or not to be” is the ultimate question. “活下去还是不活下去”是首要问题。
2. I must be a kapok beside you. 我必须是妳近旁的一株木棉。
3. She promised to be a film star. 她有希望成为一名电影明星。
英英解释
n.
1. a light strong brittle grey toxic bivalent metallic element
2. have the quality of being; (copula, used with an adjective or a predicate noun)
3. be identical to; be someone or something
4. occupy a certain position or area; be somewhere
5. have an existence, be extant
6. happen, occur, take place
7. be identical or equivalent to
8. form or compose
9. work in a specific place, with a specific subject, or in a specific function
10. represent, as of a character on stage
11. spend or use time
12. have life, be alive
13. to remain unmolested, undisturbed, or uninterrupted -- used only in infinitive form
14. be priced at
