The dissertation examines Plato's analysis of the grammar of sentences, and the radical revisions he introduced into his metaphysics in order to support his newly discovered grammatical insights. I begin the dissertation with an analysis of the evidence for a certain erroneous view of sentences, introduced by one of Plato's contemporaries. This view construes sentences as if the terms of which they are composed are mere names, a construal which is based on a denial of the possibility of syntax. I proceed to show that in several of his later dialogues Plato is keen to expose the detrimental results this view has for epistemology and metaphysics, results which he then proceeds to correct by introducing a proper account of syntax. I begin with the Theaetetus. I argue that the third part of the dialogue is concerned with showing that the erroneous view of sentences in question is inimical to any meaningful epistemology. In the process of showing this, the Theaetetus also begins to suggest a connection between syntax and certain key metaphysical notions. In the second chapter I discuss the second part of the Parmenides. I argue that the same view of sentences as the one examined in the Theaetetus is responsible for the paradoxical results of the first and second deductions of the exercise found in the second half of the Parmenides. I argue that the main reason why the deductions reach conclusions which contradict the hypothesis they began from is that the hypothesis itself is construed in accordance with this view of sentences. The third chapter of the dissertation deals with the Sophist. I show that in this dialogue Plato spells out the connection between metaphysics and syntax. I show how the inquiry into the nature of being, which is found in the central part of the dialogue, serves as a basis for the analysis of syntax we find in its final part. Understanding the connection between these two discussions allows us to see that, according to Plato, the surface grammar of sentences veils certain metaphysical facts which only the philosopher has access to.