As I have explained, I spent 18 months of my dissolute youth in Germany without learning German. I still feel a little ashamed every time I visit that fine country.
You might think it’s too late for me to fix this: It is widely believed that adult brains lose the language-acquisition knack. But I actually think that’s a cop-out.
Infants have various often-overlooked advantages: cognitive and social plasticity; absence of embarrassment; nothing to do all day but learn. And adults labor under the opposite cluster of properties.
But adulthood also brings benefits: mature intellectual appreciation of the task; access to sources other than unexplicated observations of conversational interaction.
Adults can learn new languages, especially given a good grounding in practical phonetics. I could learn German. If it weren’t for all the other things I have to do.
I should at least make a good-faith start on learning some of the nitty-gritty stuff that I never learned as a teenager. I may not be able to say “invest with a fief,” but I should at least be able to say the.
The German definite and indefinite articles, famously, take different forms according to number (singular or plural), case (nominative, accusative, genitive, dative), and three genders (masculine, feminine, and neuter). It’s gender that’s the real burden. Nouns are classified in a way that is partly sex-based (males are generally masculine, etc.), but not nearly enough: Austria is neuter but Switzerland is feminine; the moon is masculine but the sun is feminine; it’s feminine for a cat but masculine for a tiger; etc. For every noun, you have to learn which gender it has.
We do not find the logically possible total of 2 × 4 × 3 = 24 forms: Articles show no gender-related differences in the plural, 4 × 4 is the size of the array. But for some reason German decided to populate the 16 cells with just 6 distinct shapes: das, dem, den, der, des, and die. Each is used for either 2 or 4 of the possible gender/number/person combinations. And which shape is used for which feature combination seems almost random.
I managed to squeeze out a tiny bit of marginal subregularity by rearranging the genders in a nontraditional order (masculine, neuter, feminine), but not much, as you can see in this color-coded matrix:
Masculine and neuter are always the same in the genitive and dative; only masculine ever shows a nominative/accusative contrast; feminine is the same as plural except in the dative … But the array is just about devoid of mnemonically useful patterning. There aren’t really any general rules to learn.
Yet a fluent German user must hold all of it in permanent memory, and access it within milliseconds.
I could have learned the definite article in about 20 seconds if German had been more like EasyGerm (which unfortunately doesn’t exist):
And I could have learned it in one second if it had followed English in choosing the most learner-friendly pattern of all: one shape regardless of number, case, or gender. It doesn’t seem to hold English-speakers back, does it?
But no, in real life we get . Positively unfair. True, it has extra pay-off in that several other determinatives inflect the same way: alle (“all”), dieser (“this”), jeder (“each”), jener (“that”), mancher (“many”), solcher (“such”), and welcher (“which”). But really, that is cold comfort.
You don’t have to point out to me that infants learn very differently, acquiring tacit knowledge of the patterns through slowly becoming accustomed to the patterns in the millions of noun phrases they hear, until it’s second nature. I know that. But I don’t have the option of spending four or five years lying around listening to an indulgent Mutter und Vater who assign me no duties except learning to talk like them.
I’m going to have to learn the system in  out of a book, and learn the gender of thousands of nouns. This is not going to be a cakewalk.
Footnote: Naturally I’ve been recalling Mark Twain’s wonderfully funny rant “The Awful German Language.” If it had been properly relevant at any point above I would have cited it. It wasn’t relevant. But it sat there on my shoulder as I wrote, like a watching owl.