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Michael Halliday's Functional Theory for language change posits that language evolves in response to the needs of its users. According to Halliday, the functions of language can be categorized into three main roles:

1. Ideational Function: Using language to express ideas and convey information about the world.

2. Interpersonal Function: Employing language to establish and maintain relationships, express attitudes, and interact with others.

3. Textual Function: Utilizing language to structure and organize communication, ensuring coherence and cohesion in discourse.

As society changes, so do the functions required of language, leading to shifts in language use, structure, and meaning. This theory emphasizes that language is not static but is a dynamic tool shaped by social, cultural, and contextual factors.

I cannot provide direct sources for previous answers, but the information about Michael Halliday's Functional Theory of language can be found in the following foundational texts and resources:

• Halliday, M. A. K. (1978). Language as Social Semiotics: The Social Interpretation of Language and Meaning.

• Halliday, M. A. K. (1985). An Introduction to Functional Grammar.

• Halliday, M. A. K., & Hasan, R. (1976). Cohesion in English.

• Academic articles on functional linguistics and sociolinguistics that discuss Halliday's theories.

These works extensively cover Halliday's views on language functions and changes.

The Ideational Function in Michael Halliday's Functional Theory can indeed encompass what is termed as Experimental Meaning. This aspect involves the representation of reality through language and the grammatical resources employed to convey our experiences of the world. Specifically, it addresses the questions of who, what, when, where, and how, thus framing the way we interpret and describe our surroundings. This function allows individuals to articulate their perceptions and understandings of various phenomena in their lives, making it a crucial part of language use that reflects not just facts, but subjective experiences as well.

The term "tristatal model" in the context of language typically refers to a framework that describes language through three interrelated elements or states. These states often include: 1. Form: The structural components of language, such as grammar and vocabulary. 2. Function: The purposes and uses of language in communication, which can vary based on context and social interaction. 3. Meaning: The semantics or connotations of linguistic expressions, encompassing how ideas and experiences are represented. This model emphasizes that understanding language involves examining not just its form but also function and meaning, highlighting the complexity of language as a tool for communication.

Lexico-Grammar: Refers to the relationship between grammar and vocabulary in a language. It encompasses the semantics and connotations of linguistic expressions, highlighting how grammatical structures shape meaning and how vocabulary conveys nuanced ideas.