The purpose of this section is not to provide a complete and detailed discussion of every kind of phrase and clause in the English language. It is, rather, a place to turn for some practical explanation that will help writers avoid certain kinds of grammar errors. It should also help students understand better what their teachers are trying to tell them if both sides use the same terms. This page should assist teacher/student communication regarding sentence skills.
A phrase is simply a group of words that go together. There are several types of phrases, but not one of them can be a sentence all by itself. Phrases add dimension to our sentences; they provide details. Phrases can contain many combinations of words and be quite long (so don't use length to determine if something is a sentence or not). What phrases don't have is both a subject and a verb. They may have a noun; they may have something that looks like a verb; but they won't have both. This knowledge is important for avoiding sentence fragments, one of the major errors.
For a detailed discussion about the different kinds of phrases and how they can be used, consult your handbook.
Like a phrase, a clause is also a group of words that go together. The difference is that a clause has both a subject and and working verb. The group of words has to have both a subject and a working verb to be a clause. If it's missing one or the other, the group is a phrase. But it gets better. In English we have more than one type of clause--the independent and the dependent.
The independent clause is called "independent" because it can be lifted out of a larger sentence to be a sentence by itself. This kind of clause is not "dependent" on any other sentence part or group of words to be a sentence. It stands alone; it has a complete thought.
The dependent clause (also called a subordinate clause) cannot be a sentence by itself; if lifted out of a larger sentence, it cannot stand alone as a sentence. It is "dependent" on an independent clause to be part of a complete idea.
Why is this important? Well, if you can't distinguish between a phrase, a dependent clause, and an independent clause, you will have trouble with writing fragments instead of sentences. Sentence fragments are confusing for your reader. Avoid them.
For more information about how various dependent clauses can be used in a sentence, consult your handbook.