pair 后面跟单数还是复数?pair 后直接加名词怎么办

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.

要看具体情况了。
一般是复数

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...

复数