Question

They not watch TV when their parents arrived

Answer 1

They were not watching TV when their parents arrived.

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Answer 2

When their parents arrived, they were not watching TV. This means that at the specific time when their parents came, they were engaged in some activity other than watching television. It could be assumed that they were doing something else, such as playing a game, doing homework, reading, or maybe even engaged in a conversation. The fact that they were not watching TV at that moment indicates that they might have been occupied with another task or had simply turned off the TV.