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1. The window was looked at by Mary.

Does #1 sound not OK?

Is there any sentence that sounds OK including is/are/was/were looked at?
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Sorry, 1. does not sound OK.

Mary looked at the window.

Mary looked through the window.

Mary looked out of the window.

John
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Many verbs do not or only rarely appear in passive voice even though it is possible to construct such, moon. The verbs of sensation are some of them.
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Comments  
The passive voice is more common in formal English, especially if the meaning is to 'consider'. Here is an example: The proposal was looked at by the Council.
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