If I say something can be changed, for example, how would I say that in Latin? Would I say id X potest, or is there some other construction for this? The context doesn't really matter, but it's for my Commentarium Latinum entry about drawing people.
Yes, there is a passive infinitive. If something can be loved, seen, pulled, or heard, you can say amari/videri/trahi/audiri potest. In the first, second, and third conjugation you just replace the final -e of the active infinitive with -ī. In the third conjugation you replace -ere with -ī.
I interpreted your question to mean that you wanted a present passive infinitive. The active and passive infinitives also have past and future forms. Some of these forms require an auxiliary verb (esse or iri). If you want to know more about different infinitives, please consult a Latin grammar and ask a new question if something is unclear.