Well, you’re right and everyone else who answered to say “German’s not hard” is wrong because you quite properly noted that its the rating on the Foreign Service Institute’s list. This is the agency that teaches U.S. government employees new languages for their assignments.
A Category I language isn’t terribly hard - it probably has a lot of characteristics similar to English. For example, German still uses the Roman alphabet, so that’s a plus. It also has a lot of cognates - words that are similar to English words and mean the same thing.
However, when you look at the list, “Dutch” and “Afrikaans” are on the easier side, while German is right up there with Spanish, which is a Romance language with no direct link to English. So what gives?
Well, German and English started to drift apart in the 12th century when English stopped creating new words out of existing words (compounding) and started borrowing words from Norman French.
Next, English dropped word endings for noun cases and used syntax of Subject-Verb-Object. German went with the syntax Subject-Object-Verb. By comparison, most European languages went Subject-Verb-Object too - German is the outlier.
Then German has an unusually high number of articles (usually indicating gender, but also case) that don’t exist in English or even French.
And then there are prepositions. English and French have some outlying prepositions (in a car, on a plane) but in German prepositions kind of come at you randomly.
So let’s do some comparisons
English: Good evening. How are you? Have you spent much time here in New York City? It’s been a pleasure to meet you.
Dutch: Goedenavond. Hoe is het met je? Heb je veel tijd hier in New York City doorgebracht? Het was een genoegen u te ontmoeten. [Okay, not too bad - “spent” comes at the end of the sentence, but otherwise everything is pretty much the same despite SOV order)
French: Bonsoir. Comment ca va? Avez-vous passé beaucoup de temps ici à New York? Ce fut un plaisir de vous rencontrer. [Grammar is pretty much the same, vocabulary is quite different, but manageable]
German: Guten Abend. Wie geht es dir? Haben Sie viel Zeit hier in New York City verbracht? Es war mir eine Freude, Sie kennenzulernen.
Here, we can see German has fewer cognates than Dutch (“Abend” not “avond”, “geht” not “hoe”). Then we run into “you”, which in German gives you three choices depending on how formal you want to be. “Sie” is one of the more formal ones, but as you can see the Dutch are more familiar, using “je” which is more of a cognate. Then there’s “meet you” which in Dutch is “ontmoeten” but in German is “kennenzulernen”.
