optimisim 乐观  例句：The candidate's optimisim about his chances of winning encouraged his supporters. 候选人对于获胜机会的乐观态度，鼓励了他的支持者。rack one's brain绞尽脑汁，努力思索  例句： Mr. Brown racked his brain trying to remember where he left the wallet.布朗先生拼命思索，想记起他把钱包放在什么地方了。 I can't leave sb. by himself.我不能把某人单独留下。 对话：A: Would you like to go to the concert?B: I'd love to. But I can't leave my son by himself. I've to take care of him at home. 　A：你想去听音乐会吗？ B：我想去，但是我不能把我的儿子单独留下，我得在家照看他。