The few solid theories of second language acquisition existing today, from among hundreds of them, often share basic tenets that grant them as viable. These are assumptions based on social, biological, and cognitive facts that welcome the plausibility of alternatively similar facts occurring within an SLA context. The validity of the theory comes mostly from its research-based sources, as well as for the strength of the philosophical foundations that prompt the rationale of the research.
a) School of thought- A hypothesis that may become a theory in SLA must base its research on either a cognitive or a behavioral framework. If a theory does not part from the behavioral/cognitive baseline then the research will hardly be substantiated.
b) Learning is inherent- All theories of SLA ultimately say the same thing: the student WILL and CAN learn the L2. Some theorists say that learning is done by the linguistic corpus of the LAD; some say it comes out of habit formation. The important thing is that all theorists must firmly believe that all students have a capacity for learning, whether it is a high or a limited capacity.
c) Language has personality- Language cannot be studied in isolation. It has prosody, pragmatics, and context. It is three-dimensional, ever-changing, and entirely based on the user, more so than on its own rules of usage. Keeping this basic assumption is imperative because the strongest theories of SLA and learning have emphasized over and over in the need to teach in context and not in isolation.
d) Duality- The theoretical assumption of SLA is that both languages will be acquired and only the one which is spoken the most will prevail and become dominant. No matter what is your natural language it is not birth, but exposure what grants the ability. The more it is done the more effective language usage will become. This basic assumption, when added to a new theory, strengthens its philosophical foundation. The dual nature of acquisition in L2 is a well-researched fact.
There are more assumptions but these are the most important because research has backed them over and over with millions of participants of different age groups who present a number of variables. Most of the theoretical foundation of SLA has been built upon these main assumptions.