The way babies acquire their mother language is taken by the communicative methodology of learning and acquiring the target language, English in our case, up to some extent.
They receive a day-long input from their relatives, moms for example, who treat their babies with a great affection. All this helps to acquire a language.
Alike, those relatives, moms and dads in the first place, make their babies repeat phrases or sentences, more phrases and words than long sentences.
Little by little those babies acquire the language, but before speaking they receive the massive input. This latter is rather long, and then, after 2 years, depending on each baby, he or she starts to speak.
Krashen said babies have a potential of grammar capability that is innate. / Photo from: MOTHER-AND-BABY oxygenhotel hu