pair of quadratic forms

例句：

1. 1. The man and woman are a married couple.
   这对夫妇是夫妻。
2. 2. She has two children, a boy and a girl.
   她有两个孩子，一个男孩和一个女孩。
3. 3. They bought a pair of shoes for their son.
   他们给儿子买了一双鞋。