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 participal 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 participal in an adjectival sub-clause and no longer a simple adjective.