"That boy is drinking."
Translation:Tamten chłopiec pije.
Well, "tamten" is the furthest demonstrative in Polish, in meaning equivalent to 'yonder', but since that is now gone from the English language(excluding one dialect of British English, AFAIK), it is translated as 'that' usually.
"tamtem", on the other hand, is not a word in Polish. ;-)
The thing is, that Polish doesn't really use 'tamten', 'tamto' etc. as often as English. They are used really more as "that cat over there". It's a different notion of closenes.
So, Polish uses "ten/ten/tamten", while English uses "this/that/that". So the second 'ten' and first 'that' overlap. That is why "ten" is accepted as a translation of "that". Only accepted though, not suggested; it's not a translation, rather an interpretation.
And to add a bit of an longer explanation to Jellei's post, I once wrote a detailed comment about it:
"It should also be noted there is a incompatibility in conceptual distance between Polish and English:
Polish: ten; ten; tamten
English: this; that; that
So, eng. "this" is always "ten" in Polish, but pol. "ten" can be eng. "this" or "that", while pol. "tamten" is always "that".
This can possibly be made less confusing when you compare historical versions of those languages:
Middle Polish: ten; ów; tamten
Early Modern English: this; that; yonder(yon)
In Polish, demonstrative "ów" merged with "ten", but in English "yonder" merged with "that".
(also, note that "ów" is still sometimes used in literature and some Polish dialects/gwaras, but usually no longer carries the meaning of not as close as "ten", but not so far as "tamten", AFAIK)"
And that's pretty much the explanation of the situation – „tamten”(and its keen) really means "yonder", but since it's no longer used in English(excluding South US and one dialect of BrE), it is translated to "that", but that is only an approximation.