All five sentences are grammatically correct, but they don't all mean exactly the same thing. The differences come down to emphasis, tone, and clarity. 1. He resigned as manager after eight years. This is the most natural and commonly used version. It clearly means he stepped down from the role of manager after eight years. The phrase "as manager" tells us the specific position he resigned from. 2. He resigned as a manager after eight years. Also correct, but slightly more general in tone. Saying "a manager" suggests he may have been one of several managers, or it may not refer to a specific managerial position. It's a bit less direct than sentence 1. 3. He resigned the position of manager after eight years. This version is more formal. It means the same as sentence 1 but sounds more official or written, such as in a report or announcement. 4. As a manager, he resigned after eight years. This sentence is grammatically fine, but a little unclear or vague. It tells us that he was a manager at the time he resigned, but it doesn't clearly state that he resigned from the manager role. It's possible he resigned from something else while holding the position of manager. 5. As a manager, he resigned his position after eight years. This version is clearer than sentence 4. It makes it more specific that he resigned from his position as a manager. It's also slightly more formal in tone. In summary: All are correct, but sentence 1 is the clearest and most natural for general use. Sentence 3 and 5 are more formal. Sentence 4 is less specific and could be misunderstood.

In #3, I would capitalize "Manager" since it is the proper name of the position he resigned. If you turned it into an adjective, you wouldn't need to capitalize it: "He resigned his managerial position." Alternatively, you could use the noun as an adjective without capitalizing it: "He resigned his manager position." #1 and #2 are acceptable but become more clear if you change the word order: "He resigned after eight years as Manager (or 'as a manager'). I don't like #4 or #5 because these sentences don't say what he resigned from. He resigned while he was a manager - that is clear - but what did he resign from? I know that seems strange, but consider this example: Jack, the manager, performed many jobs at the company. He was the person in charge of hiring. He was the person who instructed new employees. He kept the company's books. It was too much for him to do, so he resigned his position as instructor of new employees.

Thank Mr Dan very much.

Sentences 1 & 2 are fine. I would say sentence 3 has a problem: change it to "he resigned FROM the position..." Similarly, for sentence 5, say "he resigned FROM his position" Sentence 4 seems like it could be worded much clearer, but depending on prior context, it could be acceptable. They all have similar meanings, with slight differences that will be negligible in some contexts but significant in other contexts.