Drag and drop question

Hello

I am trying to create a drag and drop exercice where the object being dragged will be accepted by the receiving object on the basis of its attributes (or some other conditions perhaps defined by variables attached to it). If it is rejected, then it should be sent back to its original position on the page.

here is an example. I would like students to construct grammatically correct AND meaningful sentences by dragging words to a "category" box, e.g. "subject" or "verb" or object.

Dealing with verbs is easy but subjects and objects are not so easy.

.For example:

The boy (acceptable subject dragged to the subject slot) eats the sandwich (dragged to the object slot) - gives a grammatically and semantically acceptable sentence.

The sandwich (unacceptable subject) eats the boy (object) - gives a grammatically correct but semantically unacceptable sentence.

This would be easy enough if that's where it ended - I could specify categories. However, I also want to be able to have sentences from the same list of objects like:

The sandwich (acceptable subject) tastes great.

In this context "the sandwich" is now an acceptable subject.

Had I been using a standard computer language, one could attach any number of variables to these words to control acceptability - but I am lost in articulate. I  hope this makes sense.

Any clues would be really welcome.

Thank  you

Andrew