A clause is a special type of phrase whose Head is a verb. Clauses can sometimes be complete sentences. Clauses may be main or subordinate.
Traditionally, a clause had to have a finite verb, but most modern grammarians also recognise nonfinite clauses.
- It was raining. [single-clause sentence]
- It was raining but we were indoors. [two finite clauses]
- If you are coming to the party, please let us know. [finite subordinate clause inside a finite main clause]
- Usha went upstairs to play on her computer. [non-finite clause]
A clause is a structure which expresses a situation such as an action, process or state of affairs. One or more clauses can make up a sentence. For example, He will find them is a main clause which stands alone as a sentence. By contrast, that he will find them is a subordinate clause functioning as a Direct Object within the main clause I know that he will find them.
The National Curriculum's definition of a clause as "a special type of phrase whose Head is a verb" presents us with a new way of looking at clauses. This may take some getting used to. However, the definition makes sense if you view a clause as a grouping of words in which one word stands out as pivotal, and this is the verb. This idea isn't really any different from regarding a group of words whose pivotal element is a noun as a noun phrase, and a string of words whose main element is an adjective as an adjective phrase.