A phrase is a sequence of two or more words, acting as a unit in a sentence.
Compare these two examples:
It is deep
His never-ending love for Mary, his old girl-friend, is remarkably deep
The pronoun it replaces the long stretch his never-ending love for Mary, his old school-friend. This is proof that they can function in the same way when we form sentences, but obviously the long phrase is much richer in meaning. We also understand that a single it is suitable only if the text has mentioned his love for her before (see 4 - the text level).
His never-ending love for Mary, his old girl-friend is a noun phrase; remarkably deep is an adjective phrase.
The head word in a noun phrase is a noun or pronoun; in an adjective phrase it is an adjective; in a verb phrase a lexical verb, in an adverb phrase an adverb. The four lexical classes can all be expanded into longer phrases. Here are examples with the heads in bold.
The reception at the Conference Centre (was a success)
All facts which point in the same direction (must be considered)
John’s first world championship title (was won in 1998)
Somebody from Belfast (told me)
...can’t accept (the decision)
...could have been punished (more severely)
have… been talking…. (…they…about their plans?)
…very strange (it proved)
….remarkably deep (his love is)
….quite keen to try the medicine (they are)
…rather disappointed that nothing had been done (people feel)
…very seriously (they took it)
…immediately afterwards (it was found)
curiously enough (nobody objected)
One more phrase type needs to be mentioned: the prepositional phrase. It consists of a preposition + a noun phrase. It does not have a head.
under the carpet (I found it)
from 2004 (….it has been normal practice)
by smashing it open (she managed it …….
When we combine phrase types to form clauses, we express statements or ask questions. The material in parenthesis in the examples above gives you the full clause. We will now move on to the clause level.