Ground coriander vs coriander seeds
Ground coriander is made from the seeds, so you definitely can substitute seeds. But you do need to grind them first. It's hard to get them ground very fine by hand, at least for quick cooking dishes, but if you toast then before grinding they're more brittle (so break up better). In a longer cooking dish the bits soften a little, so you can get away with slightly bigger bits. 

Whole coriander seeds (or big pieces of them) are unpleasant to find in food, and don't deliver their flavour well to the dish. They're too big and hard to ignore, but to small to go round (unlike the cardamom pods sometimes found whole in rice dishes).

Seeds store well, keeping their flavour, so toasting/grinding them when you're ready to use them makes for a better-tasting end result. If you're used to using ground coriander that's been hanging around a while, this will be particularly noticeable, and you want want to use a bit less.