Why is the following sentence ungrammatical? *Mary and a hammer broke the window?

The sentence is technically grammatically correct. That being said, consider these two grammatically correct sentences:

"Mary and Tim broke the window."
"Mary and a hammer broke the window."

The first sentence is perfectly valid. Two people broke a window together. Now take a look at the second one: Yes, Mary can break the window, but do hammers break windows? I believe the confusion here is because hammers do not actually perform the action of breaking windows, rather people break windows with the aid of hammers.

The sentence, though not ungrammatically correct, could definitely be worded differently. Hope this helps!

-A