Singular means one. Plural means two or more than two.
Some plural nouns are irregular. They do not follow the simple rules of adding s or es. They follow different rules or they do not follow any rules at all, so you need to remember each of them.
For nouns ending in f and fe, change “f/fe” to “v” and add -es to make them plural.
one knife -- two knives
one half -- two halves
For nouns ending in us, change “us” to “i” to make them plural.
one syllabus -- two syllabi
For nouns ending in o, add -es to make them plural.
one potato -- two potatoes
Some nouns are the same in both singular and plural form.
one deer -- two deer
Some nouns change a few letters or complete spelling.
one man -- two men
one child -- two children