English is an affluent language as it retains the flexibility of forms and the multiplicity of messages conveyed through such forms. Phrases and sentences which are the two forms of linguistic structures can serve a number of grammatical and communicative functions. A grammatical function of language refers to the role of phrasal forms in a sentence as a subject, a predicator, a complement and an adjunct. A communicative function denotes the purpose of using the forms as the sentences. The purposes may be to advise, request, order, invite, greet, warn, thank and so on. A key purpose of this article is to expose form- function relations in English. The article writer mustered some phrases and sentences as the nominal data from the books on communication in English. Phrases and sentences which are considered forms in the study are analyzed from the perspectives of their grammatical and communicative functions respectively. This article winds up with a striking remark that a phrasal form can serve a number of grammatical functions and a sentence form can serve numerous communicative functions and vice versa. Furthermore, there is not always one to one correspondence between a linguistic form and its grammatical or communicative function.