Theoretical linguistics is the branch of linguistics which inquires into the nature of language itself and seeks to answer fundamental questions as to what language is; how it works; how universal grammar (UG) as a domain-specific mental organ operates, if it exists at all; what are its unique properties; how does language relate to other cognitive processes, etc. Theoretical linguists are construct models of linguistic knowledge, and developing linguistic theory. [1]
The fields that are generally considered the core of theoretical linguistics are phonology, morphology](https://en.wikipedia.org/wiki/Morphology_(linguistics)), [syntax, and semantics. Although phonetics often guides phonology, it is often excluded from the purview of theoretical linguistics, along with sociolinguistics. Theoretical linguistics also involves the search for an explanation of linguistic universals, that is, properties that all, or many languages have in common.
References
- Theoretical Linguistics, wikipedia.org