English is a complicated language.  It's rather annoyingly difficult in many respects, and somewhat simplified in others.  Many people have difficulty with English spelling.  The grammar has it's ups and downs, but here is some help.  This series is for those who would like to brush up on or learn about grammar and grammar terminology.

But English nouns have no cases, except maybe a form of the Genitive, although most don't consider it to be genitive but prefer to call it Possesive.

The difficulty in English Nouns lies in the plurals.  However, most nouns are regular and thusly fairly easy.  The common ending is -s or -es.  The phonetic endings are -/s/ -/z/ or -/ɪz/ ( or /əz/ depending on the dialect).  The first ending, /s/ occurs when the word ends with a devoiced sound, cats /kæts/, locks /laks/ (in my dialect), maps /mæps/, beliefs /bilifs/.
Whereas the /z/ ending is when we have a voiced consonant or a vowel at the end.  Words like logs /lagz/, flies /flajz/, minds /majndz/, cells /sɛlz/, jams /dʒæmz/ etc etc.
The /ɪz/ ending occurs when the word ends with /s z ʃ tʃ/ or the letters s, z, sh, ch (or tch), and x.  They add the vowel in so that it is easily recognized.  This is also where -es appears.  This is in words like boxes /baksɪz/, witches /wɪtʃɪz/, bushes /bəʃɪz/, passes /pæsɪz/, etc.

Now with these rules, one might think English nouns are a piece of cake.  But it really isn't because we quite a few irregular nouns.  And these are often the most common in daily speech.
There are nouns like man /mæn/ and men /mɛn/.  Then there is one of the weirder ones, woman /wəmən/ and women /wɪmən/.  then we have those which don't change at all like fish /fɪʃ/, deer /diɹ/, and sheep /ʃip/.  Then we have a few other irregular nouns as well. We have wacky ones like die and dice, mouse and mice, child and children, etc.  There are a lot which end with /f/ that become /vz/.  Words like knife and knives, elf and elves, dwarf and dwarves, etc.  Many also involve some vowel shifts like man and men, foot and feet, tooth and teeth.  There are also a bunch of Latin based ones like fungus and fungi, radius and radii, etc.

Then we have the genitive.  Where we add 's to the noun.  So the Teacher's eyes are the eyes of the the teacher.  Then we can have plural which is just adding an ' to the plural that ends with s.  So you have a regular word like teacher, then you have teachers, teacher's, and teachers'.  That's how it works with that.  Many of the irregular ones form it in an even easier way.  Man, men, man's. and men's.  It seems to stay true for most irregular nouns.