I was just wondering re the following.
If one is using an eruv on shabbos, is it only permitted to carry things that one requires for use on Shabbos or can one carry items he does not require?
To give an example, if someone borrows some plastic plates from a friend to use for their seuda, and then gets home to realise they have sufficient plates, would one need to use the items he has carried?
What you are asking is not really related to using an eruv, as carrying within it is permitted. Your question is regarding the issue of hachana, that we don’t do things on Shabbos in order to prepare them for after Shabbos. The reason being that it isn’t respectful to the Shabbos that it is used as a means to prepare for other days. In your instance you did not bring the plates to your house in order to use them after Shabbos but for Shabbos itself, therefore it isn’t considered preparing it for the weekdays, even if in actuality it wasn’t used for Shabbos.