they introduce a (or several) new "structures" each chapter. so one chapter might have basic verb conjugation "i eat, he eats, etc", then something like "there are", and they will add maybe 5 new verbs each chaper (to eat, to speak, to find, etc).
then later on they teach past and future tenses, such as "i will leave", "i was visiting", "i visited". then theres conditional stuff, like "I could", subjunctives, relative pronouns, antecedents, imperetives, adverbs auxillary and participles, imperfect, modal, tenses, indirect and relative pronouns, prepositions, genders of nouns...not to mention vocabulary.
...then you realize how much you've taken speaking english for granted! But really, its not hugely difficult at all, i just wanted to demonstrate the depth that one needs to eventually learn to become fluent in any language.