1. Michael Halliday developed systemic functional linguistics, which views language as a social semiotic system. It considers how language evolves based on the functions it needs to serve in communication.
2. Systemic functional linguistics analyzes language through three metafunctions - the ideational to construe experience, the interpersonal to enact social relations, and the textual to combine the other two into coherent texts.
3. Halliday's theory is based on five principles - paradigmatic choice, stratification of meaning, the three metafunctions, syntagmatic structure, and instantiation between system and instance. It provides a framework to explain the complexity of human language use.