I've heard that in imap, when I delete things, they're not truly deleted, that I have to "empty trash" - how do I do that?
Correct: the standard IMAP protocol marks
items for deletion, but doesn't actually delete them until you take further action.
How to actually delete depends on your mail program. In Thunderbird, you can use the "Empty Trash" command after right-clicking on the trash folder. If the messages are still in the main folder, but they're marked as deleted, then right click on the folder and choose "Compact."
Other mail programs work similarly. You can do a web search for "<mail program name> imap expunge deleted messages".