Prečo by nemohlo byť správne aj „Nikdy tady nekupujeme mléko.“ ?
My si zde nikdy nekupujeme mléko ?
Added. There are hundreds of currently accepted forms for this sentence and it is very difficult to keep track of them.
Dobrý den. What function does the si play in this variant? It seems like it must be one of those clitic thingys, but if so, which other word does it depend on?
Here it is not an integral part of the verb, but it is possible to add. It means to buy for yourself, it can be viewed almost as the dative case "sobě".