A clause is a group of words that contain a subject and a predicate. A clause may be either a sentence (an independent clause) or a sentence-like construction within another sentence (a dependent or subordinate clause).
Tom married Amy when he was 19.
The string Tom married Amy could be a complete sentence on its own; the additional string, when he was 19, could not be a complete sentence on its own. It is a clause. A clause is a sentence-like construction contained within a sentence. The construction when he was 19 is ‘sentence-like’ in the sense that we can analyse it in terms of the major sentence elements (subject, verb, etc. . . .). It has its own subject (he), it has a verb (was), and it has a subject complement (19). In addition to these major sentence elements, it has the subordinating conjunction when, which tells us that the clause is a subordinate clause.
There are two major types of clause
Independent Clause
A group of words made up of a subject and a predicate. An independent clause (unlike a dependent clause) can stand alone as a sentence, beginning with a capital letter and ending with terminal punctuation such as a period. It is also known as a main clause.
Examples:
The next morning he hung around the Store until we returned from church.”
In an era when the average man was about five feet tall, the new emperor stood six feet four
Dependent Clause
A group of words that has both a subject and a verb but (unlike an independent clause) cannot stand alone as a sentence because something about it implies that there is more to come. On its own, a dependent clause is left hanging, its meaning incomplete. It must be combined with an independent clause in order to form a complete sentence. It is also known as a subordinate clause.
Example
My fake plants died because I did not water them.
If the speaker had said “because I did not water them” the listener would have wondered what or who the pronoun, “them” in the clause represents but the addition of a main clause “My fake plants died” gives the sentence a complete meaning.