Many languages assign genders to all nouns. In czech, nouns are masculine, feminine, or neutral, and the masculine category is further divided into animate and inanimate. The gender of the noun can make a big difference throughout the sentence, and the rules for how to form the seven cases of each noun vary by gender and by whether the word ends with a hard or soft sound (although there are special cases for certain word endings).
Plural is another story, with the noun changing depending on how many things there are (in most cases there are three forms: singular, plural up to four, and plural five and up).
For pronouns, where English retains a few vestiges of declension (e.g., I and me), Czech is much more complex as well. So when I used a sentence with “my eyes” in it, and very carefully selected the form of “my” to match the plural of “eye”, I was taken aback to be corrected. “Hang on,” I said, flipping to the relevant table in my textbook. “I want to make sure I have this right. Oko is neutral, right?”
Here’s the thing. The singular for ‘eye’, oko, is neutral. The plural, oči, is feminine. The same goes for ears and children.