German grammar
Yep, German has declension of articles/adjectives, which disappeared from English hundreds of years ago. Because German was my second foreign language, after Spanish, I already had a systematic way of organizing the grammar in my head. What linguists call "mature language skills". Declension made perfect sense to me, even though it took some memorization.
What was harder was memorizing gender and plural each time I learned a new noun. The feminine nouns are easy to spot, but distinguishing between neutral and masculine can be difficult if one has never encountered the noun before. And, as you pointed out, sometimes genders don't make sense. "The girl" is "das Maedchen" which is neutral (sorry the board isn't supporting umlauts today). Mark Twain summarized it best, in his famous essay, see link below.
My first foreign language was Spanish, which was required in California schools (makes sense). Spanish has a fairly simple grammar. The hardest thing, as you pointed out, was to realize that we "do" conjugate verbs in English, except the endings are all the same except third person singular. Most people who grow up speaking English don't know what conjugations are, and they think the third person singular is just an irregular spelling exception (that's what I thought until age twelve....).
German was at university, which required competence in German, French, or Russian for all candidates for a science degree. They didn't care if you could speak or write the language, but they wanted you to be able to read a scientific paper in one of those languages (this was in the 1970s, before "everything was written in English"). There were two ways to meet the requirement:
1. take a normal course that taught all four skills: reading, writing, listening, speaking.
2. take a special "reading only" course that only taught one to read.
I chose to take the normal course, as it seemed more practical for travel or other non-academic reasons. Only one year of study was required, but I earned high marks and opted to continue for two more years. I won an award sponsored by the German Consulate in Boston (and paid for by your parents' tax money!!). It was a set of books, not money. No, Helmut Schmidt did not personally present the award.
I should add that because most of my university classmates were from the Midwest or Northeast, most of them met the French, German, or Russian requirement since they had studied one or more of those languages in secondary school. The students mainly affected by the requirement were from the Southwest (Texas, California), where the study of Spanish was more prevalent. At the time, French was the most commonly studied language in the USA, but Spanish has since replaced French as #1.
I have enormous respect for anyone from Germany, Holland/Belgium-Flemish, or Scandinavia who masters French or Spanish. Even more so if they attempt it before learning English. ALL of the words are "foreign" in this case. At least English speakers recognize half the words when they study either Germany or Spanish or French. As you alluded, this may explain why fewer Germans attain proficiency in Romance languages versus English.
Another phenomenon I have noticed is in Sweden. I have friends my age (50-ish) who were taught in Sweden to emulate an OxBridge pronunciation (Standard Received Pronunciation heard at Oxford and Cambridge). To some degree, this form of English is still the goal in Germany (in NRW, if you use American spellings in secondary school, you are marked incorrect). In Sweden, kids grow up watching non-dubbed movies and tv in English. There were some US tv programs in Sweden when my friends were kids, but maybe once or twice a week. Now the airwaves are saturated with US television and movies. The result is that my friends' kids all speak with American accents, even ones who have never set foot in North America. They have acquired American pronunciation from movies and television.
Then again, Americans who study German are taught "Hochdeutsch" or "Buhnenaussprache"(stage German). This is wonderful for watching television news or documentaries on German tv. However, outside of Hannover, we are painfully aware that no one in Germany actually speaks like this, so out on the streets, where people don't enunciate every word without a dialect, like the tv announcers, it's an entirely different world.
I still remember running some errands in Straubing-an-Donau with my friends' very young daughters (six and five years old). The shopkeepers could understand my questions, but I had no idea what they were answering back to me, and I think they were TRYING not to use dialect (i.e. that WAS their version of Hochdeutsch). The two little girls, who spoke Hochdeutsch at home with their NRW-born parents, would translate the answers from dialect into Hochdeutsch, and then I understood.
en.wikisource.org