pair 后面跟单数还是复数?

pair自身可以是单数或者复数.但是pair of 后面的名词必是复数：a pair of shoes.
就是一把钳子也是：a pair of pliers；
一条黑裤子：a pair of black trousers.
pair后面直接跟名词组成词组时,要根据具体情况来定.如果是定语性词组,根据规则名词用单数：
Pair Card Game
The system is modeled by the Hartree-Fock description of a pair electron interaction.
如果是名词性词组,则单、复数要根据句子的重点来定：
The player places a base pair card from his hand into the sequence.
In case when a winning combination or a pair cards are drawn,these cards will be held automatically.
Lone pair electrons have a stronger repulsion than bonding pair electrons.