Hledejte na webu a najdete dobrá vysvětlení https://www.helpforenglish.cz/article/2012082801-sometimes-vs-sometime
Yes, if you want to stress he does indeed buy it here and not like the other shops where he explicitly does not.
Or when you want to stress that he buys it here and not borrows or steals...
so saying it like that is similar to saying that he always buys it here ... and no where else?
No, the point is what constutes the new information in the message, the thing that you want to inforrm about and that is the word "kupuje". You know about the milk already but you are stressing he BUYS it here or that he DOES BUY it here. It is the comment.
Google Translate says that:
never = nikdy
sometimes = někdy
Is this correct? How can I distinguish each one? They sound very similar...