A study in San Diego, California, in 1990, demonstrated that language acquisition can happen outside the classroom. In fact, sometimes it takes place in a large tank of water.
When dolphins and orcas were placed together, some of the dolphins actually learned to make orca sounds. And they didn't just reproduce the sounds without context; they were actually able to communicate. One of the dolphins was able to repeat a sequence of sounds back to the human trainer, even though the trainer had not been working with that dolphin at all. The trainer had taught the sequence to an orca, who taught it to a dolphin, who repeated it back to the trainer. Three species intra-communication.
Do you think they used grammar books? Was the memorization of verb tables involved? Or did they just talk?