play fast and loose

fast and loose是一对反义词，fast为“紧”，loose为“松”，两者都是副词。从字面上看不出play fast and loose是什么意思。但至少fast和loose这一对反义词给了我们一点启示，即“时紧时松、经常变化、缺乏一贯”。正是如此，play fast and loose是一个表示某些人的作为和处世态度的成语，有些人对友谊、对工作、对政治、对爱情不是始终不渝，而是朝三暮四，一日一变，这样的做法即为play fast and loose，当然，这是一种很不可取的处世态度：

Mary’s mother warns her that the boy with whom she has been going out recently has a bad reputation of playing fast and loose with the affection of girls.
玛丽的母亲警告她说，近来常和她一起出去的那个男孩对待女孩子的感情朝三暮四，名声不好。

The two political parties both refused to admit him for he had played fast and loose with either of them.
两个政党都不愿意接受他，因为他对两个党的忠诚都不专一。

give (free) rein to

rein指的是套在马身上的缰绳，驾车的人拉着缰绳便控制着马的前进方向，因此，rein便含有control的意思，give (free) rein to somebody的意思是“把缰绳交给某人，让某人自由地去驾驭”，意即“把控制权交给某人，让某人按自己的意愿去自由地行动”，有时它甚至可以具有“放任、纵容”的意思：

Our principal is very liberal-minded. He gives free rein to the teachers in the way they teach and evaluate their students.
我们的校长十分开明，老师怎么教，怎么考，他都由他们自己去决定。

Do you think it advisable for parents to give free rein to their teenage children so as to develop their individuality?
你觉得家长对十来岁的孩子条取放任态度，以让他们发展个性这种做法可取吗？

When left alone in her room in the evening, she stared out of the window at the starry night sky and gave rein to her imagination.
当夜晚来临，她独自一人留在房中时，她仰望窗外繁星密布的夜空，听任她的思想自由地驰骋。

sour grapes

如果把这个成语直译成汉语，我想你也会理解它的含意的，因为“酸葡萄”这一带有特殊意味的说法已被汉语吸收了。这个成语源出于一个著名的伊索寓言。传说一个大热天，一只狐狸又热又渴，它一抬头，正好看到了葡萄藤上结着一串又大又熟的葡萄。狐狸想去摘，但个子不够高，连续跳了几次，仍然摘不到。它无可奈何，只能悻悻离去，自我安慰地说道：“那葡萄是酸的，我才不要吃呢!”：

She said that Jane’s new coat was old-fashioned and not worth the money she had paid for it. I think it is a case of sour grapes because I know she had been longing for such a coat all the time.
她说简的新大衣式样陈旧，不值得她付出的钱。我看她是买不起才这么说的，因为我知道她一直想要一件这样的大衣。

John said he was more interested in the competition itself than in the prize. Obviously it was sour grapes to him.
约翰说他对比赛本身比对奖品更感兴趣，很明显，对他来说，奖品是可望而不可及的东西。