The plural endings for masculine nouns can take on various endings in the plural form. (s) = singular. (p) = plural. (d) = definition.

Some nouns end in -e : der Arm (s) = die Arme (p). (d) = arm/arms

Some end in -en : der Junge (s) = die Jungen (p). (d) = boy/boys

Some end in an umlaut + e in the plural form : der Ball (s) = die Bälle (p). (d) = ball/balls

Umlaut + -er ending : der Mann (s) = die Männer (p). (d) = man/men

Some end in only an umlaut : der Vater (s) = die Väter (p). (d) = father/fathers

Some have no change in the ending : der Spiegel (s) = die Spiegel (p). (d) = mirror/mirrors

And now the plural ending for feminine nouns:

Can end in -en : die Zeitung (s) = die Zeitungen (p). (d) = newspaper/newspapers

Can end in -nen in the plural (especially for nouns ending in -in) : die Amerikanerin (s) = die Amerikanerinnen (p). (d) = American (woman)/Americans (women)

Can end in umlaut + -e : die Nacht (s) = die Nächte (p). (d) = night/nights

Can end in just an umalut in the plural : die Mutter (s) = die Mütter. (d) = mother/mothers