a pair of
組になっているもの→単数扱い
A pair of sunglasses is what you'll need there.
２つ一組のもの→複数扱い
A pair of dolls were given to me.
a pair of の代名詞は they （複数形）
I bought a pair of pants and I gave then to my brother.
this pair of shoes = these shoes と表現できる
組になっていないもの
a couple of pencils
