Singular means one. Plural means two or more than two.
Some plural nouns are irregular. They do not follow any rules, 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