However, the things that make a language hard to learn for kids aren't necessarily the same that make it hard for adults.
The article you link is about acquisition of past tense formation in Danish, where some verbs have a past tense ending in -ede, like bade/badede (bathe/bathed) and some others end in -te, like kalde/kaldte (call/called) and yet others are completely different. Those verb endings are frequently reduced in speech, which makes them sound similar or identical. In the study, children who used more different reduced forms were more likely to use a -ede form for a verb that usually goes with -te.
An adult learning a language is more likely to learn the "proper" form in a controlled environment first and then learn to slur their verb endings later when they reach a level where they can have casual conversations with native speakers. Compare also English "would of" vs. "would've," which native speakers seem to confuse a lot, although they're obviously different to someone who first encountered those words in writing.
Aren't children supposed to be better at learning new languages than adults, though? There's supposed to be a critical period where language acquisition is much easier.
It’s not like they are better it’s just repeating everything a million times and talking the whole day without shame of making a mistake is more fun when you are 6.
The fact that vocabulary of a 6 year old child is much smaller and takes less time to remember of course helps
On the other hand, when you are adult , you can actually understand abstract concepts of grammar. It’s just too boring to repeat everything enough times to train your neural network to get it right without thinking.
What children are actually really good at are sounds of language.
The article you link is about acquisition of past tense formation in Danish, where some verbs have a past tense ending in -ede, like bade/badede (bathe/bathed) and some others end in -te, like kalde/kaldte (call/called) and yet others are completely different. Those verb endings are frequently reduced in speech, which makes them sound similar or identical. In the study, children who used more different reduced forms were more likely to use a -ede form for a verb that usually goes with -te.
An adult learning a language is more likely to learn the "proper" form in a controlled environment first and then learn to slur their verb endings later when they reach a level where they can have casual conversations with native speakers. Compare also English "would of" vs. "would've," which native speakers seem to confuse a lot, although they're obviously different to someone who first encountered those words in writing.