German learner mistakes cluster predictably around gender, case, and word order. Knowing them in advance makes them far easier to catch.
Bare nouns make later case-and-article agreement much harder — see German Vocabulary Guide.
Starting a sentence with a time expression but then placing the verb third instead of second, carrying over a habit from less strict word-order languages.
Dropping the prefix at the end of a clause changes or loses the verb's actual meaning — see German Verbs.
A compound noun takes the gender of its final element, not its first — a common, avoidable mistake.
Forgetting that subordinate clauses push the verb to the end, unlike main clauses.
Pronouncing a final "d" or "b" as voiced when it should devoice to "t" or "p" at the end of a word.
Saying "twenty-one" in English order instead of German's "one-and-twenty" (einundzwanzig) pattern.
Sidestepping verbs that require the dative case rather than learning the specific common ones directly.
Treating ä, ö, and ü as decorative rather than genuinely distinct sounds that can change meaning.
Avoiding conversation until gender and case feel fully mastered delays real fluency more than the mistakes made along the way would.
See also the reference page: German Grammar.
Learning nouns without their gender attached, which makes later article and case agreement much harder than building gender in from the very first encounter.
Forgetting verb-second order after starting a sentence with something other than the subject (like a time expression), carrying over habits from less strict word-order languages.
Yes — the prefix carries essential meaning, so omitting it at the end of a clause changes or loses the verb's actual meaning rather than being a minor stylistic choice.
Saying numbers in English order ("twenty-one") instead of German's reversed units-before-tens pattern ("one-and-twenty," einundzwanzig).