Monkeys can do simple grammatical rules but not rules with hierarchial structure
“For example, the monkeys could master simple word structures, analogous to realising that “the” and “a” are always followed by another word. But they were unable to grasp phrase patterns analogous to “if… then…” constructions.”
(actually, in the study, the grammar that the monkeys could do was “A is always followed by B”, and what they couldn’t do was “Repeat A for some number of times, and then repeat B the same number of times”)