Can I wear a crew neck jumper over a shirt?

Yes. The shirt collar shows above the neckline, creating a classic smart casual outfit. A thinner, smooth knit works best.

How do I wash a men's jumper so it does not stretch?

Wash it at a low temperature on a wool or hand-wash cycle and do not wring it hard. Dry it flat, not on a hanger, because wet knitwear stretches. Always check the care label.

Crew neck or V-neck jumper: what is the difference?

A crew neck sits closer to the neck and looks more casual, and it works well on its own. A V-neck shows more of the shirt and tie, so it is more often worn in office outfits.