A clause is a key grammatical structure and this means that clauses are things that you need to have, at the very least, a basic grasp of. Thought of at its simplest, a clause can be considered as a short 'sentence' - one that occurs either on its own (e.g. "I ate the jelly") or together with other clauses to make a longer sentence (e.g. "because I was hungry").
A clause, then, is a group of words that is either a whole sentence or is a part of a sentence.
Clauses are built up from individual words or from small clusters of words called phrases.
Most clauses are built around a main verb which tells, often, of an action, thought or state, e.g. "I ate the jelly because I was hungry".
A clause can be what is called independent. This mean it is acting as a simple sentence, as in the example, "I ate the jelly". Independent clauses can also exist as a part of a larger sentence when they are called not an "independent clause" but a main clause.
Another common type of clause exists just to help out the meaning of a main clause. This second kind of clause is, therefore, dependent on its main clause for its meaning. An example would be the dependent clause, "because I was hungry"; you'll see here that there is an extra word at the start of the clause: "because". It is this extra word that stops the clause being able to be independent or to be a main clause; the word "because" forces the clause to be dependent on some other main clause, e.g. "I ate the jelly because I was hungry". This words acts to subordinate its clause and so is called a subordinator. Subordinators create dependent clauses - more often, these days, called subordinate clauses (sometimes reduced to "sub-clauses"). There are many subordinators.
Look at this example: "He hit him even though he was a friend":
He hit him MAIN CLAUSE even though he was his friend. DEPENDENT (subordinate) CLAUSE
An important kind of clause acts as if it were an adjective - it adds extra information about a noun or noun phrase. These clauses are called relative or adjectival clauses. They can seem confusing because they can be inserted in between their main clause, e.g. "The girl who wore a red dress left early." This sentence contains one main clause "The girl left early" and one dependent or relative clause, "who wore a red dress".
The subordinator in this example, the word "who", is acting as a pronoun (i.e. it is a word that takes the place of, and stands in for, a noun). Here it is called, therefore, a relative pronoun because it introduces a relative clause.
Other relative pronouns are "that" and "whom".
Sometimes the relative pronoun can be missed out to create an elliptical relative clause, e.g. "The joke [that] he told was funny"; here the relative clause is "he told".
The structure of clauses is fairly fixed in English syntax (S = subject V = verb O = object C = complement A = adverbial). In certain dialects and in poetry the syntax can be varied and the sense still kept, e.g.
S+V+O: Alison / sang / a song. O+V+S A ballad /Alison / sang. S+V: Alison / sang. S+V+C: Alison / is / a good singer. S+V+A: Alison / sings / in the choir. S+V+O+O: Alison / sang / her mum / a ballad. S+V+O+A: Alison / sang / the song / from the song-book.