Because it doesn't match the noun.
Polish determiners (this, that) have forms dependent on the gender (and also the case). So in Nominative you will for example have "ten chleb" (this bread, masculine) but "ta kaczka" (this duck, feminine).
Off topic, but I'd like to congratulate you and the team on the frequent use of Present Continuous. This is almost non-existent in the English from Polish course (and in all the Duo English courses that I've seen), resulting in a lot of somewhat unnatural constructions.
Easy to understand the rule, except the fact of duck being considered feminine. In my language (Portuguese - Brazil), we use masculine pronoun to reference to a duck.
There's no better answer than 'it just is'. Every species of animal has its name and it can be either masculine, feminine or (rarely) neuter.
In most Slavic languages ducks are considered female, at least it's valid for Russian, Ukrainian and I guess Belorussian too, we have another for words to mention a male duck, the same with hens, but not with geese, it's consisted male, although we have 2 other words to talk about a male goose and a female goose, but they are used not too often
Got it marek, can you please tell me howmany nouns are there, if you explain me like that it would be great help, dzienki!
How many nouns? Do you mean how many genders?
Well, in singular you have masculine, feminine and neuter. There is one case (Accusative), where it is also important whether a masculine noun is 'animate' or 'inanimate'.
In plural you have 'masculine personal' plural (almost only 'groups including at least one man') and 'not masculine-personal' plural (as the name says... basically anything else. dogs, boxes, women, houses, everything without men).
I hate it when words that are different genders in different languages. In French the duck in general is a male unless specified. Is there a word for male duck in Polish?
Yes, "kaczor". "Donald Duck" = "Kaczor Donald".
But we generally don't accept other words for animals than the 'default species name'. It may rather create confusion when suggested to the learner, than be helpful. And frankly I don't think you'd hear "kaczor" often outside of conversations about the citizens of Duckburg (Polish: "Kaczogród", if anyone's interested).
it's said that "ta" can be translated as "this, the, that". but the lesson says that "ta" is used just for nearby things.
Well, if you look just at the literal meaning, then "ta" = "this" and therefore it's just for nearby things. However, Polish and English determiners differ in usage. They differ in terms how they perceive 'closeness'. Basically, Polish [ta/ta/tamta] (and forms) are equivalent to English [this/that/that]. The middle forms overlap. "tamta" is really more like "that one over there" than just "that". The main translations in this course are always the direct ones (this=ta and that=tamta), but every form of "ta" should indeed accept [this/that/the].
When users refer to "the lesson" where exactly are these lessons? The app seems to only have excercises with transition hints