A simple definition of a sentence is: a set of words that expresses a complete thought and contains a subject and a predicate. Let's look at this.

The subject is the person or thing performing the action that the sentence is talking about. Everything else in the sentence is the predicate. The predicate must include a verb - the word that describes the action. It may also include an object - a person or thing that is being acted upon - and any other phrases.

The shortest sentences are all verb; for example, Help! or Duck! In these one-word sentences, the subject (and the object, if there is one) is understood; if the sentence Help were written in full, it would read You help me. Usually, however, all the elements in the sentence are expressed: I love you madly consists of subject, verb, object and 'other (adverbial) phrase'.

Finally, the sentence must contain a complete thought. This is best demonstrated by considering an incomplete sentence: Although the car crashed through a barrier. This has a subject, "car", a verb, "crashed" and a predicate, "crashed through a barrier". If you left "although" out, it would be a sentence. But "although" signals that the writer has more to say; the thought, and therefore the sentence, isn't finished.

Incompleteness is one of the two most common faults in sentence construction.But the other, bigger, problem is that some writers don't know when to stop. We all know that we start a sentence with an initial capital and end it with a full stop. An amazing number of people get to the end of their sentence, toss in a comma, and keep going on to another sentence. I wrote all night to finish the assignment in time for my deadline, in the morning, I could hardly keep my eyes open.

Once you get to the end of your complete thought, stop. Or, if the ideas in two sentences are closely linked, join them together with a joining word like 'and' or 'but', or (if you're really brave) a semi-colon. (If you'd like to use semi-colons but are not sure of the rules, see Punctuation pauses.)