Student struggles with the passive

I've been tutoring a Latin student lately. His mother informs me that he's got trouble with the passive. It's not a problem of understanding—he gets the passive in English. It's not a problem of recognition—he recognized passive verbs in Latin. The problem is mapping the Latin passive onto the English structure, and I think I've got the problem nailed down.

Here's a passive sentence in Latin:

Fenestra frangitur.

Here is one way to say that in English:

A window is broken.

Except that's not what any actual English speaker, let alone a 14 year-old English speaker, would say. More like:

A window's broken.

Which is exactly one phonological segment off of:

A window's breakin'.

And I think that's the key. Students hear the progressive (is verb-ing) and some, but enough to screw thing up, passives (is verb-en) as the same thing. So to eliminate the ambiguity they hear, the students kill the tense:

A window was broken.

I could be wrong, but this seems to be the student mistake with present tense passives. Anyone have any insights?