Knit one purl one

Actually the title is completely misleading because this has nothing to do with knitting. Here is the actual puzzle.

Pull one.

Cost two.

Get off one.

Off the top of one.

Keep one up.

Bite one.

Drag two.

Cross two.

Note I shall add some easier ones as time goes by unless someone gets it first.

Question

(a) What sort of thing are these "ones" and "twos"? —- —– (4, 5)

and

(b) What common linguistic phenomenon are they part of? —— (5)