Determining the plural form of nouns isn't always easy. Do you remember when Harry said the following?

Example:
Harry: Gibt es in Deutschland viele Staue?
Anna: Du meinst Staus? Ja, leider gibt es viele Staus in Deutschland.

Harry chose a plural form that was in fact wrong, but could be right in theory because most monosyllabic masculine nouns take an "e" at the end to form the plural. You've already learned that there are five different ways of forming the plural of a noun. Three of those ways include the additional possibility of adding an umlaut as well.

1. Plural ending –e or –e + Umlaut

2. Plural ending –en or –n (if the last letter of the noun is already an –e)

3. Plural ending –er or –er + Umlaut

4. Plural ending –s

5. No ending in the plural or no ending + Umlaut 

So which endings to use for which nouns? There are a few general rules, but they don't apply in all cases. So there are many exceptions. Nonetheless, they can help you to a certain extent:

The plural of monosyllabic nouns are frequently formed by adding
-e to the end of masculine nouns: der Mord – die Morde
-en to the end of feminine nouns: die Frau – die Frauen
-e or -er to the end of neuter nouns: das Jahr – die Jahre, das Schild – die Schilder

The plural of many loanwords is formed by adding an "s" to the end:
der Job – die Jobs
das Taxi – die Taxis

Nouns that end with "e" often get an "n" to form the plural:
der Name – die Namen
die Torte – die Torten

Nouns that end with "er", "el" and "en" frequently take
– no ending for masculine nouns: der Bankräuber – die Bankräuber
-n for feminine nouns: die Zwiebel – die Zwiebeln
– no ending for neuter nouns: das Zimmer – die Zimmer