"In June, the days are long here."
Translation:V červnu tady jsou dlouhé dny.
Again the famous flexible Czech word order. NOT! I'm sure there is a rule but all people can say is that it sounds unnatural. Please help!
That word order sounds quite unnatural. But "Dny jsou tady v červnu dlouhé" or "V červnu jsou tady dny dlouhé" are OK.
"V červnu tady jsou dlouhé tu dny?" Why is "the" understood here but required at other times?
Certainly not "tu dny", but we can discuss "ty dny".
Well, Czech does not have articles and uses demonstrative in other situations than English uses articles. Sometimes these coincide, sometimes they do not. I think here specifically you might be confused by the change of the sentence structure.
The closest (not necessarily natural) translation to the Czech sentence "V červnu tady jsou dlouhé dny." would be something like "In June, there are long days here.". Not even English has any "the" here so it is natural to have no demonstrative in Czech.
The most direct translation to "In June, the days are long here." would be something like "V červnu jsou tady dny dlouhé." Now again, no demonstrative, but at least "ty dny" sounds just unnatural here, not as a complete nonsense. I could perhaps even imagine some context where would indeed use "ty dny".