I read your comments to Bobby's answer and would like to disagree with him, to some extent. You are just beginning to learn English, and it is better to start learning CORRECT grammar now, instead of trying to unlearn bad habits later. It is true that the vast majority of English speakers (including Australia and the UK, as well as the US) don't always follow the strict rules of grammar in their everyday speech. In your casual chats with English speakers, you won't have to worry about using perfect form -- clear and understandable communication is the main goal. But I have worked with 100s of Chinese college students who ... YES ... must pass certain tests in English to achieve their goals. If you and I were chatting, and I said, "我是好“， you would correct me, would you not? And I would be grateful for your help, because I don't want to sound like a 老外 who doesn't respect your beautiful 普通话. So I think it is great that your teacher stresses proper English, AND that YOU care enough to question her about things you don't understand. I TRULY commend you for that! Now, about your question ... The two phrases ('he watched...' and 'he was watching ...) are basically identical in their meaning, but depending on the CONTEXT, one or the other might be preferable. Example 1: > A: "What was he doing yesterday while you were studying?" > B: "He was watching TV all day." Example 2: > A: "What did he do yesterday?" > B: "He watched TV all day." It is true, you could use either 'B' answer to either 'A' question, and you would be understood clearly, AND you would be grammatically correct. But can you see how each answer form above is a good match for the question as it is stated? Perhaps the context of your lesson is what led your teacher to say one answer was preferred. Don't get frustrated, Nancy!! The fact that you are here on italki, asking questions, is a sign that you are a good student who wants to learn what is correct. Keep up the good work! 加油！

Both of these are correct and would be well understood by the person you said them to. Try not to get too caught up in the grammar of things , unless you are trying to become an English professor. As a native English speaker , I would understand both of those things and view them the same.

You could say "He watched TV all day yesterday" and "He watched TV all day yesterday".