This thesis studies the grammar of complement clauses, is finite and infinitive clauses in nominal function, in the English of the Wyclifite Sermons c1400 (Select English Works of John Wyclif, I & II ad T Arnold, Oxford 1869-71), and characterises their occurrence in a corpus of c60,000 words. The complement system is fully described, partly within an informal transformational framework and with historical or more general linguistic justification as necessary, and an appendix lists constructions found with individual matrix verbs. Noun phrase complementation is generally involved, and the infinitive with subject behaves in many ways as a connex clause. There is particular discussion of the factors controlling infinitive marking, and the syntactic isolation of modal auxiliaries. Within finite clauses the oppositions between direct and indirect speech, indicative and subjunctive, inflectional subjunctive and shulde, and between clauses with and without main clause word order freedoms are given special attention, as is a use of how to indicate a following summary or narrative. The factors which control the presence of at in clauses of indirect statement and question are also discussed. Verbs of thinking and declaring are beginning to take nonfinite clause objects as the result of a series of minimal alterations to English structure made with Latin as a model. This results in a distributional pattern which parallels that found today, and which, it is hypothesized, is controlled by the same general factors. In certain negative contexts kat-clauses regularly contain an initial ne of real or pleonastic negation. These are investigated in detail, and a semantic hypothesis about their grammar is advanced, as well as an account of their development showing the interaction of the influence of Latin quip and native structural pressure.