Michelson was awarded a Nobel Prize in physics for the work--the first American so honored--but not for twenty years.
I'm not clear about what the phrase 'but not for twenty years' mean. Is it saying that Michelson was awarded a Nobel Prize in physics for the results of the work but not for his twenty years of work?
Thank you in advance.
It was 20 years later that he received the award. He did not receive the award until 20 years had passed.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.