1.
Stress: putting *emphasis* on certain words
Pitch: Whether you talk in a squeaky high girly voice or a nice manly one
Intonation: how your pitch goes up/down as you talk (note that aussies speak in a monotone pretty much, so this only really applies at the end of sentences when you ask questions, make a statement, tell a story (HRT-you'll learn that) when it rises/falls)
2. Open class words readily accept new members, eg nouns, verbs, adjectives all fall under this
Closed class words don't get new words/change too often, eg pronouns, conjunctions, prepositions. If these changed too quickly you'd have trouble understanding english.
3. Not 100% sure any more (grr, 100 things you learn in unit 4 you instantly forget). My definition I found in easy words: "2 words that often occur together as part of a set phrase" (although it could be more than 2, *slaps self*) Definition I found in handout "Collocations are combinations of function and content words that frequently occur together". Egs from handout I found: salt and pepper, blue-eyed blonde o_O
If it helps, co-location