"Your mouse is eating cheese."
Translation:Wasza mysz je ser.
Is "mysz" a feminine word? I understand there are exceptions, but my understanding is that it would be "wasz mysz je ser"
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)