Language Acquisition theory is quite extensive but, when it comes to early acquisition during infancy, the main theory is easier to break down into three subtheories, which are identified as: a) imitation, b) reinforcement and, c) active construction.
Imitation is understood easily enough and its process is the easiest to promote. It consists on the acts of listening and extrapolating specific sounds and blends, processing them internally, and then re-producing them externally. Imitation is key in Second Language and First Language Acquisition (SLA/FLA) because it constitutes the first step of the process; after all, if you cannot hear, you cannot properly speak. A clear example of imitation occurs in children who interact with others and end up speaking just like them. In adults, a good example of imitation comes in the form of a few celebrities who suddenly "adopt" a new accent in order to fit in. Madonna comes to mind during her marriage to Guy Ritchie, when she was publicly laughed at by the press for trying to speak in British English since her husband was also British.
Reinforcement consists on learning language through conditioning. Language usage is monitored and rewarded as it progresses. This is often the case in SLA courses such as EFL, ESL, FLES, and ESP. Think of a classical scenario where children are learning to speak Spanish or English as a second language and conduct daily drills and practices to get rewards from their teacher. Similarly, in a first language scenario reinforcement happens daily during school hours, which is where conditioning occurs the most.
ACGT, or active construction of a grammatical theory, is also a conditioning process where level 2 language students (slightly more advanced than first learners) continuously get information on how to use the language most appropriately and to their advantage. Adults learning L2 are active users of ACGT because they mostly acquire second languages for specific purposes (ESP/SSP/LSP). Therefore, it is imperative that the modeling and monitoring of appropriately-used language is ever-present in the SLA process in adults.