Six is correct. Five and 3 could be correct, but probably not. For is used to indicate a continuous action. It does not indicate an action that was completed at some time in the past. So: 1. I bought this book 3 weeks ago. 2. He graduated from university five years ago. 3. The have been gone from their hometown for decades OR They left their hometown decades ago. 4. My poor pet died one week ago. (It would have been quite poor if it took him a whole week to die). 5. The Smiths have moved to the suburb for half a year. This could be right, if you mean that they have moved for 6 months only and then will return to the city. Probably this is not what is meant, so you would say: The Smiths moved to the suburb 6 months ago.

Thanks！I know the usage of for!

1. I bought this book 3 weeks ago. I have had this book for three weeks. 2. He graduated from university five years ago. He has been out of university for five years. 3. They left their hometown decades ago. They have been away from their hometown for decades. 4. My pet died a week ago My pet has been dead for a week. 5. The Smiths moved to the suburbs five years ago. The Smiths have been living in the suburbs for 5 years. 6.Tom has known Mike for 15 years...CORRECT Tom met Mike 15 years ago.

I think it's great that you are interested in learning about present perfect with 'for.' You already know one important part of this structure: use 'for' with a length of time, e.g. for ten years, for a long time. There is one more thing you need to know about present perfect with 'for.' You can only use it with verbs that show a state, or with actions that have duration. Sentences with this structure answer the question, "How long?" So it can only describe actions that continue for a long time. You can't use it for actions that start and end quickly. For example, your sentence #6 is correct: "Tom has known Mike for 15 years." This is because you can know a person for a long time. On the other hand, your sentence #1 is not correct: "I have bought this book for 3 weeks." It takes only a minute to buy a book and then the action is finished. You cannot ask, "How long did you buy the book?" Buying is a short transaction. Instead, you should say: "I have HAD this book for three weeks" or "I have OWNED this book for three weeks." For the same reason, sentences #2-5 are also incorrect. You must choose verbs that have duration. Graduate, leave, die, and move are all short actions that are completed in the past. You cannot ask, "How long did he graduate?" Instead, you can say: 1.I have HAD this book for 3 weeks. (Or: I bought this book 3 weeks ago.) 2.He has BEEN OUT OF SCHOOL for 5 years. (Or: He graduated five years ago.) 3.They HAVEN'T LIVED IN their hometown for decades. (Or: They left their hometown decades ago.) 4.My poor pet has BEEN DEAD for 1 week. (Or: My poor pet died one week ago.) 5.The Smiths have BEEN IN the suburbs for half an year. (Or: The Smiths moved to the suburbs half a year ago.) 6.Tom has known Mike for 15 years. (OK!)