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All employees of 40 years and over who want to participate in the workshop must submit their application forms by Thursday.

Q1) Is this sentence grammatical and natural? I think this sentence is grammatical and natural.

Q2) What does "who" refer to in this sentence? I think "who" is referring to "all employers"

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The sentence is correct. "Who" refers to employees that are 40 and over and want to participate in the workshop.

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All employees of 40 years and over

Natural English is All employees 40 and over.

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Comments  

Ambiguous. "Employees of 40 years and over" means employees who have been there for 40 or more years. Evidently this is not intended. The presumed intent is "employees 40 years old and over." But that is only the presumption. That's why the sentence is ambiguous.

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