Ambiguity is a common problem in ESL communication that impacts meaning and leads to many misunderstandings and sometimes the logical fallacy of ambiguity. Learning to identify ambiguity and tease out possible meanings are important critical thinking and communication skills. But issues surrounding critical thinking and ambiguity in communication can be a serious challenge for ESL students as is learning how to rephrase communication so as to avoid such fallacies. This poster presents, explains, and explores a unique sentence diagramming technique as a method for teaching ESL engineering students how to identify, clarify, and avoid ambiguity as it relates to grammar and sentence structure and thus also improving their critical thinking as well. Through sentence diagramming, students learn how to identify ambiguity and the possible fallacies and misunderstandings that may result. They also learn how to clarify instances of ambiguity in their own output and in the output of others. Addressing issues of ambiguity strengthens communication and critical thinking among interlocutors. This sentence diagramming method, created by the authors, was based on Chomsky's X Bar Theory but it is not tied to it. This diagramming method is then used as a general English teaching tool to help students better understand how phrasing relates to grammar, meaning, and critical thinking. Once students understand how phrasing affects not only form and function but also meaning, they can then better understand issues around ambiguity and critical thinking. As a result, students can then more easily identify ambiguity, tease out the multiple meanings to help clarify communication, and avoid using ambiguous statements in their own communications. Sentence diagramming gives students a tool to examine similar or even identical sentences that can have multiple or easily misunderstood meanings and show how subtle differences in phrasing can change meanings dramatically. The tool can also be used when interacting with others to point out instances of ambiguity and to help clarify the intended meaning. Once students understand how to diagram a sentence, they can better identify ambiguous sentences, point out their multiple meanings, and avoid ambiguity in their own communications, thus becoming more accurate users of English and better critical thinkers.