(1) Twenty years of teaching is long enough for Mr. Taylor.
(2) Teaching for twenty years is long enough for Mr. Taylor.
(3) Having taught for twenty years is long enough for Mr. Taylor.
My non-native English speaking friends think all of my sentences are correct. As a non-native speaker, I really don't see anything wrong. What is your opinion? Thanks a lot.
(1) and (2) are both okay. Note that (1) is hostile in tone: you want to see Taylor leave. (2) is sympathetic in tone: 20 yrs.
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(1) and (2) are both okay. Note that (1) is hostile in tone: you want to see Taylor leave. (2) is sympathetic in tone: 20 yrs. of teaching has been hard on Taylor and you'd be glad, for his sake, if he stopped.
(3) is ungrammatical.