German is kind of interesting. It follows a rather rigid set of spelling and grammar rules… and compared to English, is easy to get along in. Once you know the pronunciation rules, you can say any word, even if you don’t know what it means or haven’t heard anyone else say it.
The part many find difficult is that German has three forms of the word “the”: der, die, and das, referred to as the masculine, feminine, and neuter forms. Problem is, those are just convenient three-way labels… and have nothing to do with the supposed gender of the noun in question. And they’re randomly assigned.
The only way to know which the to use is to memorize it per noun. For all of them. All tens of thousands of them.
Example: the girl == das Mädchen. (neuter form of “the”)
The problem is compounded by akkusitive and dative forms of article and adjectives. In other words, depending on the placement of the noun in a sentence (as subject, direct object, or indirect object), you might use a variation on the spelling of “the” or on the ending of a descriptive adjective.
At the end of all this, there are therefore nine (9) different forms of “the” that might be used in any given circumstance (many spelled the same). So, in written form, German can be tricky for the non-native speaker to author grammatically correct.
As far as I know, those two things (random assignment of “the” and learning to intuitively handle subject vs. direct object vs. indirect object) are the only hard parts.