One of the easiest ways to spot a non-native speaker of any language is the misuse of prepositions. German and English often use different prepositions for similar idioms or expressions: "wait for"/warten auf, "be interested in"/sich interessieren für, and so on. In English you take medicine "for" something, in German gegen ("against") something. German also has its two-way prepositions that can take two different cases (accusative or dative), depending on the situation.
HELP > Prepositional Pitfalls | Two-Way Prepositions
German Mistake 7: Umlaute
German "Umlauts" (Umlaute in German) can lead to problems for beginners. Words can change their meaning based on whether they have an umlaut or not. For example, zahlen means to "pay" but zählen means to "count." Bruder is one brother, but Brüder means "brothers" - more than one. Pay attention to words that may have potential problems. Since only a, o, and u can have an umlaut, those are the vowels to be aware of. If you need help with how to type umlauts and other special German characters or accents on your computer, see the link below.
HELP > Confusing Word Pairs in German | German Characters FAQ
German Mistake 8: Punctuation and Contractions
German punctuation and the use of the apostrophe is often different than in English. Possessives in German usually do not use an apostrophe. German uses contractions in many common expressions, some of which use an apostrophe ("Wie geht's?") and some of which do not ("zum Rathaus"). Related to the prepositional hazards mentioned above are German prepositional contractions. Contractions such as am, ans, ins, or im can be possible pitfalls.
HELP > German Punctuation
German Mistake 9: Capitalization
German is the only modern language that requires the capitalization of all nouns, but there are other potential problems. For one thing, adjectives of nationality are not capitalized in German as they are in English. Partly due to German spelling reform, even Germans can have problems with spelling hazards like am besten or auf Deutsch. You can find the rules and a lot of hints for German spelling in our capitalization lesson and try our spelling quiz.
HELP > Capitalization in German | German Spelling Reform Hits the Newsstand
German Mistake 10: Verbs with 'haben' or 'sein'
In English, the present perfect is always formed with the helping verb "have." German verbs in the conversational past (present/past perfect) can use either haben (have) or sein (be) with the past participle. Since those verbs using "to be" are less frequent, you need to learn which ones use sein or in which situations a verb may use haben or sein in the present or past perfect tense.
HELP > German Verb Review: Pres. Perfect
MORE: Also see German Mistakes 1-5.