"Moja siostra lubi ten kubek."

Translation: My sister likes this mug.

[Siroggak]

What is "tę" and what is "ten" again?

[mihxal]

"kubek" is a masculine noun so in Accusative (and actually also in Nominative) you use "ten".

"tę" is used with feminine nouns in Accustaive. For example:

Moja siostra lubi tę książkę (ta książka [Nominative] -> tę książkę [Accusative])

What means "My sister likes this book"

[immery]

when "this" more less like adjective - describes a noun
Polish word "to" has different variants depending on grammatical gender of word.
TEN is male nominative singular
TA is female nominative singular
TO is neuter nominative singular
https://en.wiktionary.org/wiki/ten#Polish

[AndrewnotinJapan]

Is there no difference in Polish between a cup and a mug? Usually in English a cup is something your granny uses (along with a saucer) and a mug is something a builder (and the rest of us) use.

[immery]

cup can be also "filiżanka"

and also puchar (for the champions etc)

I reccomend google image :)

[zagadka314]

It won't accept "that cup." I do not understand why it randomly does not accept "that" for "to" et al. Does anyone else know why? Or is this a programming mistake?

[immery]

At the beginning of the course it only accepted "this" as translation of ten/ta/to, now course contributors try to add "that" to all sentences, but they missed some. Please report when you encounter one.

[Jellei]

Added now.