What is a sentence?

A sentence is a unit of language which communicates a complete idea and is capable of standing alone. Sentences can tell you something or ask you something in a way that words or phrases cannot. In order to make a statement or ask a question, generally speaking a sentence needs a subject and a main verb.

Look at the following list of expressions:

Fred's sister
Extremely upset by the sad news
In the bathroom
Drinking vodka

All of these expressions probably conjure up pictures in your mind, but none of them communicates a complete idea. They are phrases, not sentences.
Now look at these sentences.

a) I am in love with Fred's sister.
b) Jack was extremely upset by the sad news.
c) Someone has thrown up in the bathroom.
d) Have you been drinking vodka?
e) Yesterday, my dog chased a postman into the street.
f) People who live in glass houses shouldn't throw stones.
g) If Pete invites Shirley to the party and she accepts his invitation, I think that she won't finish her project on time.

When a sentence has a subject and a main verb, and makes a statement or asks a question about an action or about a situation, it is called a major sentence. There is a small group of sentences which do not have both a subject and a main verb, and which are usually either exclamations or commands. These are called minor sentences. Here are a few examples:

What delicious apple strudel!
Get lost!
Scram!
Come over here.
Aaargh!

Typically, a minor sentence ends with an exclamation point (!).

What is a subject?

A subject is a noun phrase which typically is the topic of the sentence - that is, it names who or what the sentence is about. This noun phrase may simply be a noun. For example, in the sentence "Dogs don't like tabasco sauce", the subject is "Dogs". A single-word subject of this type would usually mean that the sentence is making a general statement. For example, "Dogs don't like tabasco sauce" refers to dogs in general rather than to any particular group of dogs.

The subject noun-phrase may also be a gerund - that is, a verb with an "ing" suffix acting as an abstract noun which names an activity. For example, "Swimming is a good form of exercise".

Noun phrases may also include articles (the/ a / an), determiners (some, all, this, that), adjectives, prepositional phrases and relative clauses.

For example:
"Dogs don't like tabasco sauce."
"Some dogs don't like tabasco sauce."
"Those savage dogs don't like tabasco sauce."
"The dogs in the garden don't like tabasco sauce."
"My pet dogs, who love pizza, don't like tabasco sauce."

Typically, the subject of a sentence appears at the beginning of the sentence, unless the sentence begins with an adverbial phrase or clause, and before the main verb.

Look again at these sentences:
a) I am in love with Fred's sister.
b) Jack was extremely upset by the sad news.
c) Someone has thrown up in the bathroom.
d) Have you been drinking vodka?
e) Yesterday, my dog chased a postman into the street.
f) People who live in glass houses shouldn't throw stones.
g) If Pete invites Shirley to the party and she accepts his invitation, I think that she won't finish her project on time.

Can you identify the subjects? Click here for the answers.

What is a main verb?

The main verb names the action, occurence or state of affairs which is attributed to the subject. In other words, the main verb states or asks what the subject does or what the subject is. A main verb includes all the auxiliary verbs which contribute to its mode or tense, and a verb phrase is typically considered to include any adverbs or complements which modify or complete the verb's meaning. You may also come across the term predicate, which means whatever the sentence claims to be true with regard to the subject. Simply put, the predicate is everything in the sentence apart from the subject.

Here are some examples of main verbs and verb phrases:

Jenny made a sandwich.
Jenny shouldn't have been making a sandwich.
Jenny will probably make a sandwich.
Jenny is extremely angry.

What is a phrase? What is a clause?
Phrases and clauses are both group of words which functions as a single part of speech i.e. as a noun, adjective, verb or adverb. The difference is that a clause will contain a subject and a main verb within it, whereas a phrase will not.

Look at this sentence:
My pet tortoise, whose name is Albert, has been hibernating in a shoebox for three months and is still extremely sleepy and bad tempered, especially if I try to make him eat lettuce leaves..

The red part is a noun phrase. It names a thing.
The first blue part is an adjective clause - also known as a relative clause. It qualifies, describes or gives more information about the thing referred to by the noun phrase to which it is connected.
The second blue part is an adjective phrase.
The orange part is a verb phrase. It names the action or state of affairs about which the sentence is making a statement.
The first green part is an adverbial phrase. It qualifies the verb, giving more information about when, where, how or why the action took place.
The final green part is an adverbial clause.

Can you identify what kind of clause or phrase the highlighted part of each of these sentences is?

a) Ever since they first met, Nora has been head over heels in love with Alfonse.
b) You should be grateful to all the people who have given you so much help.
c) I am usually fast asleep in bed by ten o' clock.
d) She tried several times, but because of her repeated failures she became more and more frustrated.
e) Despite your accusations, I swear that I am innocent.
f) Is that boy in the blue cardigan Rebecca's brother?
g) Janice has been arrested.
h) I think she has been arrested for drunk driving.
i) If anyone finds out the truth, she will be in a lot of trouble.
j) I've been under the weather recently, so I went to see the doctor.

Click here for the answers.

External Links

More on sentences can be found at these external sites:

Sentence Sense
I highly recommend this site. It offers a comprehensive course on the grammar of sentences and excellent guidance on problems with usage. Students should work through the grammar section and then use the usage section's exercises on fragments, run-on sentences, agreement and consistency.

Adverb Clauses

Relative Clauses

Sentence Fragments

Run On Sentences

Sentences: Definitions and Types