[...] if you monitor adults learning a second language, you find something completely mystifying. That German woman with her English textbook follows the exact same developmental stages as that Japanese guy with his American girlfriend. The German might progress through her stages faster—German, after all, is fairly similar to English—but she won’t skip any of them. Not only that, but both of these English students will follow developmental stages that closely resemble the development of child speech. Like the kids, they start out with -ing (He watching television) and only later learn is (He is watching). They master the irregular past tense (He fell) before the regular past tense (He jumped). Toward the very end of their development, they master the third-person present tense (He eats the cheeseburger).

These results are baffling, in part because they don’t have anything to do with the order of language textbooks and classes. English students usually encounter sentences from the last developmental stages—like “He eats the cheeseburger”—within their first week of classes. They can successfully learn to use a late-stage rule—he + eat = he eats—in the slow-paced world of homework and tests, but they invariably forget that same rule whenever they try to speak. Speech is too fast, and students just don’t have enough time to apply their grammar rules consciously. In their speech, they have to walk through each of their developmental stages in order (He eating carrot / He is eating a carrot / Yesterday he ate a carrot /He eats carrots daily). Like kids, no English students will blurt out “He eats hamburgers” before “He is eating” unless they have enough time to plan out their sentences in advance, consciously apply the right grammar rules, and say them out loud.

As far as researchers can tell, this is simply the order with which the human brain picks up English, period. And while some learners can move through these stages more quickly than others, no amount of drilling a particular grammar rule — I eat, he eats, we sit, she sits, they fall, it falls — will enable a student to skip a developmental stage. Ever.

Naturally, it’s not just English. While the developmental stages look different from language to language, every language has a particular developmental order, which children and second language learners alike will inevitably follow on their way to fluency. The most plausible explanation for these rigid, unavoidable developmental stages is this: our language machines never turn off. When we learn a second language, we develop like children because we learn like children. If we feed our language machines enough comprehensible input, then we will automatically learn our new language’s grammar, just as we did as kids.