This glossary of art terms might help, although they don't seem to offer a definition of subject matter. You could probably think about form as being to do with the physical/ visual aspects of the work - colour, shape, materials etc - content as the idea or message, and subject matter as the thing/ person etc that is depicted - eg the subject matter might be a boy on a bench, and the content is an idea about childhood; something like that. But really, different people interpret these things in different ways, so your teacher could have something else in mind!