"Your mouse is eating cheese."

Translation: Wasza mysz je ser.

[MatCiesla]

Is "mysz" a feminine word? I understand there are exceptions, but my understanding is that it would be "wasz mysz je ser"

[silmeth]

A lot of nouns ending on soft (palatal(ized)) consonants¹ or vowel /i/ is feminine, eg: noc (night), rzecz (thing), sól (salt), bogini (goddess), cerkiew (gen.sg. cerkwi, Orthodox Church), dłoń (hand)

Virtually all ending on -ść are feminine: miłość (love), kość (bone), ość (fishbone), wysokość (height), maść (ointment, rub), etc.

But there is also a lot of masculine ones: łoś (moose), pajac (pejor. clown, puppet), kac (hangover), koń (horse), stróż (guard), etc.

You can find a pretty good list of those on English Wiktionary, by looking what links to Polish declension of feminine soft consonant nouns template.

¹ ie.: j, ś, sz, ź, ż, rz, c, ć, cz, dź, dz, ń, l (and k, g, m, p, b, f, w when they get additional i in endings, like in cerkiew, cerkwi)

[MatCiesla]

Dziękuję bardzo, Silmeth!

[gabe81]

Yep, mysz is feminine, despite the lack of usual ending (-a).

[WardBogaer1]

Why is it wasza and not twoje

[Jellei]

"mysz" is feminine (it looks masculine though - it's an exception), so the right forms are either "twoja" (2nd person singular) or "wasza" (2nd person plural).

"twoje" is neuter. Or 'not masculine-personal plural'.

[RuslanLant]

Почему неправильно Twoja mysz... ?

[Jellei]

Это правильно, программа должна была принять.

[Chris827680]

In common speech, wouldn't "Wasza mysz je ser" be "Your mouse eats cheese" as apposed to is eating? I know this is all semantics but I just felt I had to ask.

[Jellei]

Most Polish verbs work both for Present Simple and Present Continuous.

Actually, for "eats", there could be (but it is not obligatory) another verb used: "Wasza mysz jada ser" (from "jadać"). But "Wasza mysz je ser" can mean both "is eating" and "eats".