Because it's the object of the verb so it has to be put in the accusative case, which for feminine nouns means replacing -a with -ę.
Duolingo should give the genders of nouns. Does anyone know a good way to get familiar with all the Polish genders?
Gender in Polish mostly doesn't need to be learned with the noun, like in French or German. For most nouns you can tell the gender by the way the word ends in the nominative singular (the form you learn):
Ends in a consonant (including 'j'): masculine
Ends in 'a': feminine
Ends in 'e', 'ę', or 'o': neuter
That covers most cases. Some of the few exceptions and things to keep in mind:
Some nouns that refer to male human beings happen to end in 'a' but are masculine: tata 'dad', mężczyzna 'man', 'kierowca' driver, sprzedawca 'salesman' ...
(Putting aside the words than logically can only refer to men - like 'tata', 'mężczyzna' - some of these words, like 'kierowca', can refer to women, but are grammatically masculine. Others, like 'sprzedawca', can only refer to men. In the plural they can all(?) refer to mixed groups... I think.)
Nouns for abstracts concepts ending in -ość are feminine: szczerość 'honesty', niepodległość 'independence' ...
A few common nouns end in other consonants (often soft consonants) but are feminine: sól 'salt', noc 'night', część 'part'...
A few feminines end in -i: pani 'ms./mrs./lady', gospodyni 'landlady'...
(This section also includes words like sprzedawczyni 'saleswoman' - the feminine version of 'sprzedawca')
Words ending in -um are neuter: muzeum 'museum', liceum 'high school'
There's probably one or two more exceptions, but they don't account for many words. The vast majority of Polish noun genders don't need to be learned: just learn the endings.
I also added the two bits in brackets after the word 'salesman' and 'landlady'. Are they right-ish?
No, I just play a Pole on the internet!
(I'm Irish, no Polish heritage, but spent years living in Poland and learned the language fairly well. Hence my uncertainy about a few little points.)