Many studies in PER suggest that language poses a serious difficulty for students learning physics. These difficulties are mostly attributed to misunderstanding of specialized terminology that often assigns new meanings to everyday terms. This dissertation presents a novel approach to the analysis of the role of language in learning physics. The approach is based on the analysis of the historical development of physics ideas, the language of modern physicists, and students' difficulties in the areas of quantum mechanics, classical mechanics, and thermodynamics. These data are analyzed using linguistic tools borrowed from cognitive linguistics and systemic functional grammar. Specifically, the idea of conceptual metaphor and grammar is used to build a theoretical framework that accounts for
• the role and function that language serves for physicists when they speak and reason about physical ideas and phenomena,
• specific features of students' reasoning and difficulties that may be related to or derived from language that students read or hear.

The theoretical framework is developed using the methodology of a grounded theoretical approach. Predictions about the relationship between student discourse and their conceptual and problem solving difficulties are tested in the context of "heat" in thermodynamics and "force" in dynamics. In each case the language that students use to reason is analyzed with the framework. The results show that language is very important in students learning. In particular, students
• use features of physicists' conceptual metaphors to reason about physical phenomena, often overextending and misapplying these features,
• draw cues from the grammar of physicists' speech and writing to categorize physics concepts.

In summary, I present a theoretical framework that provides a possible explanation of the role that language plays in learning physics, and how and why physicists' language influences students in the way that it does.