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.

pair 是指双,对,副的意思，所以一般都是要跟复数的，比如：
This pair of trousers is too tight for me.
这条裤子我穿太紧了。
This pair of shoes is durable.
这双鞋很耐磨。
又如：
I have a pair of leather mittens.
我有一副皮连指手套。
注意：主语中有the pair of 或者a pair of，,this pair of,two pairs of等短语时，谓语到底用单数还是复数，根据pair的单复数而定。即pair为单数谓语就用单数，pair为复数谓语也随之用复数。
This pair of shoes is very cheap.
Two pai