Clauses and the grammatical hierarchy
• The grammatical hierarchy is composed of the following five elements, in descending order: sentences, clauses, phrases, words, and morphemes. Although they are not at the top of the hierarchy, clauses are the most basic element and therefore most important from the point of view of a student of English grammar.

Clause patterns
• Clauses can contain five types of “functional element,” i.e. elements that are labelled according to their function in the clause. These are: subjects [S], verb (phrases) [V], objects [O], complements [C], and adverbials [A]. These five clause elements can be combined in seven ways to create the following clause patterns: SV, SVO, SVC, SVA, SVOO, SVOC, and SVOA.

• The relation between clauses and sentences
1 a simple sentence — one which does not have a clause as an immediate constituent — is also a clause. For example, the simple sentence, “Harry is fat” is identical with the clause “Harry is fat.” 2 in a complex sentence — one in which a “main clause” has at least one “subordinate” clause as an immediate constituent — once again, the whole sentence is identical with the main clause (which is its unitary constituent.) 3 in a compound sentence — one which has two or more main clauses connected by a coordinating conjunction — neither clause is identical with the whole sentence.

functions of subordinate clauses:
• Subordinate clauses can be classified according to their function as the immediate constituents of clauses: as subjects; as direct or indirect objects; as subject or object complements and as adverbials; or according to their function within clause elements: as postmodifiers in noun phrases as prepositional complements, and as the complements of adjectives. There is a further, overlapping characterization of clauses in terms of their resemblance to certain word classes: “nominal clauses” are related to nouns; “relative clauses” to adjectives; and “comparative clauses” to both adjectives and adverbs.

finite, non-finite, and verbless clauses:
• Clauses can further be classified according to whether their verb phrase is finite (containing either a simple present, simple past or a modal), non-finite (not containing a finite verb), or verbless (a “verbless.” clause does not contain a verb of any kind, but can still be “analyzed into clause elements.”