The plural is not an -s: the five routes
CEFR A1 · part 2
edumaterial's parts within the level. The European framework does not subdivide them.
The accusative only moves the masculine: der becomes denMarking the direct object by changing der to den, and checking that die, das and the…nicht or kein: the two ways of saying noChoosing kein when you negate a noun with no definite article, and nicht everywhere…Separable verbs: the piece that flies to the endSpotting a separable verb and sending its prefix to the end of the sentence when you…Reading German aloud: ei, ie, z, v, w, chReading any German word aloud, knowing which letters sound different from how they are…Every noun takes a capital letterCapitalising every noun, and using those capitals when reading to find what a long…der, die or das: the gender of nounsChoosing der, die or das, and spotting the endings that give the gender away without…The rule, in three lines
German plurals take five routes: -e (der Tisch → die Tische), -er (das Buch → die Bücher), -(e)n (die Blume → die Blumen), -s (das Auto → die Autos) and no change at all (das Fenster → die Fenster). Many also add an Umlaut to the inner vowel. And the plural article is ALWAYS die, whatever the singular gender was.
The usual slip. Adding -s to everything. In German -s is the RAREST route and is kept mostly for borrowed words. The usual ones are -e, -er or -(e)n, very often with an Umlaut: das Buch gives die Bücher, not «die Buchs».
Why it catches you out. English is almost all -s, so -s feels like the safe bet and it is the wrong one. But you have the hook already: foot/feet, mouse/mice, tooth/teeth. Those are exactly the German Umlaut plurals. Buch/Bücher is foot/feet, and sheep/sheep is das Fenster/die Fenster.