edumaterial's parts within the level. The European framework does not subdivide them.
- 1Try
- 2Learn
- 3Practise
- 4Check
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.