Answer:
Mary is cleaning the windows now. Nobody has cleaned them for a month.
The present tense is used to describe an action that is happening now. The past perfect tense is used to describe an action that happened before another action in the past.
So, in this case, we know that Mary is cleaning the windows now, and that nobody has cleaned them before that. Therefore, we use the present tense for "Mary is cleaning" and the past perfect tense for "Nobody has cleaned."
Another way to think about it is that the present tense is used for actions that are ongoing, while the past perfect tense is used for actions that are completed. Since Mary is still cleaning the windows, the action is ongoing, so we use the present tense.