The accusative only moves the masculine: der becomes den
CEFR A1 · part 2
edumaterial's parts within the level. The European framework does not subdivide them.
The plural is not an -s: the five routesForming the plural by its five routes —-e, -er, -(e)n, -s and no change—, with or…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
The direct object goes in the accusative, and of the definite articles ONLY the masculine changes: der becomes den. die stays die, das stays das, the plural stays die. The indefinite works the same: ein becomes einen only in the masculine.
The usual slip. Either thinking all four articles change and freezing, or changing none and saying «Ich sehe der Hund». Only one in four moves, and that is the whole difficulty of the unit.
Why it catches you out. This is where English has the biggest head start of the four, and almost nobody says so: English still declines its pronouns. He/him, she/her, who/whom. den is to der exactly what «him» is to «he». You would never say «I see he».