I'm a native Chinese but I don't teach Chinese, I think I can try to answer your question :) 1) 你想想再买吧 means "You should REthink (thoroughly) to buy it or not", which means the speaker may not recommend you to buy it. 你想想就买吧 is also grammarly correct but it means "You should think again and buy it". In this situation the speaker wants you to buy it. This sentence can be rude because it sounds like an order, which may be the reason why your Chinese book wrote that it is wrong. 2) 他的病还没有好,病好了以后才来上课。 (This is a correct sentence/statement) means "He's not well, he will go back to class when he's better." 他的病还没有好,（让他）病好了以后再上课吧。This sentence is also correct but it is a suggestion, which can be translated into "He's not well yet, (let him) go back to class when he feels better." 病好了以后就让他来上课吧。This is a correct sentence, using 就, it means "Let him go back to class when he feels well.", which, again, is an order. This sentence is often said by a teacher. 3) In summary, 再 means "re-", 就 can be used as an order, 才 is often used in a statement. We can discuss abt it if you have more new, different or more professional opinions. :)