1. I was studying Maths. My sister came in.
 * When: When I was studying Maths, my sister came in. (Khi tôi đang học toán thì em gái tôi vào.)
 * While: While I was studying Maths, my sister came in. (Trong khi tôi đang học toán thì em gái tôi vào.)
2. I was walking home last night. It started to rain.
 * When: When I was walking home last night, it started to rain. (Khi tôi đang đi bộ về nhà tối qua thì trời bắt đầu mưa.)
 * While: While I was walking home last night, it started to rain. (Trong khi tôi đang đi bộ về nhà tối qua thì trời bắt đầu mưa.)
3. I was studying for my exam. My phone rang three times.
 * When: When I was studying for my exam, my phone rang three times. (Khi tôi đang học bài để thi thì điện thoại tôi reo ba lần.)
 * While: While I was studying for my exam, my phone rang three times. (Trong khi tôi đang học bài để thi thì điện thoại tôi reo ba lần.)
4. I was walking to the park. I saw my friend.
 * When: When I was walking to the park, I saw my friend. (Khi tôi đang đi bộ đến công viên thì tôi nhìn thấy bạn tôi.)
 * While: While I was walking to the park, I saw my friend. (Trong khi tôi đang đi bộ đến công viên thì tôi nhìn thấy bạn tôi.)
5. They were playing basketball. It started to rain.
 * When: When they were playing basketball, it started to rain. (Khi họ đang chơi bóng rổ thì trời bắt đầu mưa.)
 * While: While they were playing basketball, it started to rain. (Trong khi họ đang chơi bóng rổ thì trời bắt đầu mưa.)