What is the difference in meaning when a past participle is used before or after the word? For example, I have put on a torn shirt, and I have put on a shirt torn. What is the meaning of these sentences?
When a participle is used as an adjective it can be considered to be an adjective and in English the adjective goes before the noun with a few exceptions (e.g. Surgeon general, sergeant major) So it would always be 'torn shirt'.
Unless as previously noted, you are elaborating on the tearing event, e.g. 'a shirt (which has been) torn by the washing machine' in which case it is a participle in an adjectival sub-clause and no longer a simple adjective.
It is frequently difficult to know when a participle is being used as an adjective or part of a verb.
The short was torn
Is "torn" an adjective or part of a verbal phrase?
I doubt that there is any objective way to determine which it is and no particular benefit in doing so.
When was the shirt torn?
The problem is the same. We create a problem for ourselves only if we insist on calling it an adjectival use and also insisting that adjective always precede the noun being modified.
The simplest rule seems to be that in questions the passive participle, regardless of whether we label it part of a verbal phrase initiated by a form of the verb "be" or an adjective modifying the subject, follows the subject. It is part of the rule regarding inversion for questions.
Furthermore, there is no rule in English that precludes post-positional adjectives; it is just not the usual word order..
We can not dedicate, we can not consecrate, we can not hallow, this ground. The brave men, living and dead, have hallowed it ...
Are we saying that this is bad English because it has post-positional adjectives?
Nevertheless, it is far from normal English to end an indicative sentence with one passive participle unless it is part of a passive verb. There may be cases when it is acceptable, but they are very rare.
The shirt was torn
It was a torn shirt
It was a shirt, torn
is not fine. It is weird.
It was a shirt, torn, begrimed, and bloody
is atypical, but is grammatical. But "torn" here is not alone in apposition. In fact, I think a single adjective in apposition is much rarer than a list of adjectives.