If someone is a know-it-all, they know everything! For example, Hermione Granger in Harry Potter would be called a know-it-all at school. And usually a know-it-all will show off (exhibit they know something really well) about their knowledge!
A Mock exam is a practice exam, so it's not the really exam. You usually get a past paper (an exam paper from the last year or before) when you sit (do) a mock exam. Curing exam times, it is always nice if someone comes through for you, which means they support you through the difficulties.
Remember to proofread (read again) your essay when you finish, and make sure you have put across (express) your idea clearly.
To know something backwards to forwards means you have learned something really well.
To have learned something off by heart means you have memorised it.

Found this websites with loads of idiom tests: http://a4esl.org/q/h/idioms.html