Grammarians are looking for categories--for sets of forms that work together.  We ask questions such as: How do words combine into "chunks" of various sorts to be used in sentences?  How are the kinds of "chunks" that we see in use different from each other?  How many kinds of "chunks" can we observe in the data that we collect? 

Phrase and clause are two basic categories and terms used by grammarians and linguists. On the whole, phrase is used to categorize combinations of words that do not have the internal structure of a sentence--no subject or predicate is involved in a phrase.  Phrases include combinations such as noun phrases, verb phrases, adverb phrases, adjective phrases, and prepositional phrases.

On the whole, clause is used to categorize combinations of words that are sentence-like.  A clause has a subject (usually) and a predicate (or we can re-analyze what is given to know that some part of the predicate has been left out to reduce the form).  For example, a simple sentence is an independent clause: I like sociology.  Or a relative clause is a clause because it started life as a sentence and has been transformed to make a clause that is attached to a noun:  I like the sociology course that I took last semester.  Or a reduced relative clause is a clause because we know what the full form would be: I like the anthropology course focused on health issues in refugee communities.  We know that the "complete" relative clause is that is focused on health issues in refugee communities--and that that complete relative clause can be analyzed as a complete sentence: The anthropology course is focused on health issues in refugee communities. 

In sum, a clause is sentence-like with something like a subject and something like a predicate.  A phrase doesn't have sentence-like features.

Unfortunately, the definitions for the two are not consistently used and eventually will lead us to problems in other areas.  What do we call an infinitive--such as to study sociology or to teach English?  Is it a phrase?  Is it a clause?  Many ESL/EFL texts prefer to call infinitives phrases because they are so different from sentences and other more complete clauses.  Many linguistics and grammar studies prefer to call infinitives clauses because an infinitive has a verb--and thus a predicate--and can have a subject.  I want you to study sociology.  The you is the subject of the infinitive--you could re-analyze the infinitive into a complete sentence: You will study sociology or You study sociology. 

That problem, however, lurks down the road.  We can start with some definitions and examples that are clear cut and generally agreed upon.
 
 

Phrases	Groups of words that work together as a unit but that do not have the internal structure of a sentence.  That is, there's no subject or verb in the group of words.
noun phrase	He took a sociology class.
verb phrase	He will have taken 10 sociology classes when he graduates.
prepositional phrase	She will take a sociology class in the spring semester.
adjective phrase: The second example shows a prepositional phrase that is attached to a noun.  Because the function of the phrase is like that of a more traditional adjective, it is classified as an adjective phrase.	The students are very happy and enthusiastic. The students in the class are working on project papers.
adverbial phrase or adverb phrase: notice that adverbial phrases can include (1) single word adverbs like late and quickly but also (2) prepositional phrases uses as adverbs.	He was late. He was late to the meeting. She ran down the hall. She ran down the hall quickly.

 
 

Clauses	Groups of words that work together as a unit and that have the internal structure of a sentence.  That is, there's a subject or verb in the group of words.
independent clause	Another name for the simple sentence.  A unit with a subject and a predicate (a verb and whatever goes with it).
dependent clause or subordinate clause	An independent clause that has been changed structurally so that it can be combined with an independent clause (as with adverbial clauses or noun clauses) or attached to a noun (as with relative clauses).
adverbial clause	Because he had limited funds, he took a bus to New York for TESOL.
noun clause	He thought that he might enjoy the bus trip. That he might enjoy the bus trip seemed a reasonable dream.
relative clause or adjectival clause	The bus that he took to NY was full of unhappy children.
appositive clause: These clauses look like relative clauses but have different internal structure. That is only a connecting word and does not have a role in the internal structure of the clause.  They are formed with nouns like belief, idea, knowledge, conviction, and so forth.  There are parallel forms noun clause sentences using the related verbs as in the second example.	We share the belief that teaching ESL/EFL is a worthwhile career. We believe that teaching ESL/EFL is a worthwhile career.

Please send me your questions and comments at patbyrd@comcast.net.  Thanks.