The research by Johnston (2010) has suggested that there are five key domains coming into play that influence children’s language development. Listed below are the five domains.

Social. Toddler’s guide their language learning by inferring from a speaker’s communicative intent (e.g. they watch what adults are looking at and referring to). Moreover, the verbal environment (e.g. verbal families that provide children with lots of input) children are exposed to between the ages of 1-3 has a positive affect on their language levels at age 9. This highlights the social nature of language and how much of language is learned through naturalist interactions with other people versus rote learning through workbooks or cue cards.
Perceptual. Ever wonder why there is such a focus on ensuring children’s hearing is tested at birth, again in kindergarten and prior to commencing speech therapy? Well auditory perception (the ability to perceive and interpret information through the ears) skills between 6-12mo of age can predict vocabulary size and syntactic (sentence structure) complexity at 24 months.
Cognitive Processes. You have probably heard SLPs emphasize the importance of repeating words to children many times. A recommendation SLPs often give is “repeat, repeat and repeat some more!” This is because frequency in input affects rate of language learning. For example, children who hear more examples of language learn language faster than children who have a lower frequency of language examples.
Conceptual. Language learning skills is affected by a child’s knowledge of the words that they are learning. In other words, children who have challenges recalling certain words tend to know less about the word they are trying to recall. For example, if a child knows what a dog is, has pet a dog, and knows what sound a dog makes, they will have an easier time recalling the word “dog” in comparison to a child who has heard the word “dog” but has no mental representation of it (e.g. unsure of what it looks like, what sounds it makes, where it lives etc.). This emphasizes the importance of not only labelling words to children but also explaining the words to them and showing them examples of the words. Books are great for this because children can be exposed to so many new words with clear examples that they otherwise might not have ever been exposed to in real life.
Linguistic. A child’s current vocabulary is something that they use to infer new word meanings. For example, if there are two objects in front of a child and one is a toy they are familiar with (e.g. a ball) and one is something they are unfamiliar with (e.g. a dice) and an adult says, “Give me the dice!” monolingual children will usually give the adult the correct item despite being unfamiliar with the term. This is because they often will use a process of elimination. For example, since they knew already what the label for ball was they can assume that is not the object the adult is referring to and will therefore in turn reach for the unfamiliar object and assume this is the “dice” the adult is talking about. However, bilingual children often cannot use this process because they are aware that the same object already often has two labels already (one label for each language); therefore, bilingual children often use other means for interpreting item labels and cannot use this mutual exclusivity principle.

References

Johnston, J. (2010). Factors that influence language development. Encyclopedia on Early Childhood Development.