"The cat puts the mouse into the milk."
Translation:Katten lægger musen ned i mælken.
In this case, both "lægger" and "putter" is correct. But generally, "lægger" is much more used because it is not as specific. "Putter" is mainly used for putting things into something, like a pocket or a hole. Or when adding something into something else, like adding flour in your pancake batch.