How does it only take a year or so to speak/understand our first language but it’s so much harder to acquire another language?

Question

Like, we have a frame of reference (first language), so why is it so hard to learn another language after that?

In: Other

Answer 1

The premise of the question is basically not true. Kids don’t learn a language in one year. It takes many years. They know two-word sentences in one year, more complex ones in the following years, and their language skills typically improve when they start kindergarten, where they spend multiple hours a days doing constant interactive exercise with their language.
Adults would learn a language equally well in that condition.