An independent clause is a sentence. A sentence is a group of words that contains (most of the time) a subject and a verb and expresses a complete thought A. A ‘complete thought’ is an idea that can stand alone- it does not require additional words for it to make sense.
I want to go home now. (sentence- a complete thought)
When is the next movie? (sentence- a complete thought)
If I were married. (not a sentence- not a complete thought)
The example above, ‘If I were married’, contains both a subject (I) and a verb (were) but it is not a complete thought. The condition ‘if’ is not satisfied. Something must be added for the group of words to make sense. ‘If I were married, I’d be happy.’ The addition of the words ‘I’d be happy’ completes the thought of this group of words.
Most of the time a sentence requires a subject and a verb, but not always. Most exclamatory sentences contain only one word- a verb.
Stop! Now! Look out!
As can be seen there is no subject in these sentences. ‘You’ is the implied subject. Within the context of a conversation the idea of ‘Stop’ would express a complete idea.
Sentences can be classified as declarative, interrogative, imperative, or exclamatory, depending on the purpose or function of the sentence.
Declarative sentences are sentences that make statements.
Barry is upset because his wife crashed their car
Imperative sentences are sentences that make commands.
Take me home.
Exclamatory sentences are sentences that make interjections.
Interrogative sentences are sentences that ask questions.
How much is this?
For more information about sentences takes a look at
An explanation of the four types and functions of sentences: declarative, interrogative, exclamatory and imperative– www.understandinggrammar.com