German nouns have genders, which are entirely non-existent in Modern English. A noun can be masculine, feminine or neuter. Note that grammatical gender has nothing to do with biological gender, it's just an arbitrary label. For example, "Mädchen" (girl) is neuter, "Zeit" (time) is feminine and "Tisch" (table) is masculine, so you'd refer to "girl", "time" and "table" with "it", "she" and "he" respectively. "Die", "das" and "der" are three different words for "the". Which one you use depends on the gender of the word. You just have to memorise the gender of each word individually.
German also has four cases: nominative, accusative, dative and genitive. English distinguishes between nominative and accusative for pronouns, but otherwise cases are entirely non-existent in English. Which word you need to use for "the" depends not only on the gender of the noun but the case it's in. Dative is not particularly hard to grasp, the problem is simply knowing when to use it (which can actually be a big problem). "Genitive" is completely alien for English speakers and by far the hardest to grasp, and probably for Mandarin speakers as well - English and Mandarin express possession the same way.
German adjectives also decline. This means that you may have to add "e", "en", "es" or "er" to the end of adjectives, depending on the case and gender of the noun they attach to. Declination doesn't exist in English at all.
German conjugation is also much more complicated than in English. Except for "to be", English verbs only ever conjugate in the third person singular in the present simple. German verbs have different conjugations for different persons and in different tenses, which you will have to memorise.
Overall, I think you'll find German harder than English, but don't let that dissuade you.