I studied German at school and it was a mess. The language has several illogical concepts which cannot be understood by foreigners, just learned by heart.
1) Genders. There are 3 genders: Masculine, Feminine, and Neutral. And only God knows what is what. For example: "boy" is masculine, but "girl" is neutral. WHY? No one can say for sure. Just learn it. In contrast to French, when 95% of words of the femenine gender end with -e, in German, there's no logic.
2) German declension (or cases). As you may know, there are 3 articles (der, die, das) for each gender + die for plural nouns. And there are 4 cases (Nominative, Genetive, Accusative, Dative). And each case changes an article and an ending of a noun. For example: "der König der Löwen" = The Lion King. Basically, König is masculine (der), but Löwen is the plulal of der Löwe. But the article for plural form of der Löwe is die Löwen. The usage of der in "der König der Löwen" means that the case is used. Without it, the phrase "der König die Löwen" means literally "the king the lions", which is a mistake, and a German person can ask WTF do you mean.
And such sh*t is everywhere, in every sentence. So, you have to learn a table of changing articles, where there are 3 main aforementioned articles plus 3 new additional. And the whole table has no logic to find and acknowledge. It looks like all articles have been randomly shuffled.
Check it. I wanna kill myself when I see those tables.
3) The work order. In some situations, the usage of complex tenses forces a speaker to change the word order. For instance, in perfect tenses.
Ich habe Englisch studiert = I have English studied. The verb goes to the end of the sentence. It's pretty simple when a sentence is short, but in complex sentences the word order makes a lot of pain in da arse.
4) Complex nouns. The word "der Schreibtischdrehstuhl" contains 4 words: Schreib (to write) tisch (a table) dreh (to turn) stuhl (a stool). What do you think the whole word means? A stool with an attached turnable table. And Germans feel ok to create nouns containing 5-6-7-8 words.
5) Regional dialects. In contrast to English speaking countries, where a person from Australia can easily understand a person from Canada or the UK (the main difference is in pronunciation and slang), some German dialects differ gramatically. People of South/North Germany, Bayern, and Swiss could face some problems while communicating with each other. Immigrants who study Hochdeutch (the generalised dialect of German TV and education) are completely f*cked up.
Just my observations based on 8 years of hardship in school.