In order to be able to punctuate correctly, it does help to have an understanding of sentences, clauses and phrases. Very briefly, a sentence needs to contain a finite verb, i.e., a verb that has a subject. A clause also has to contain a verb: a main clause can stand on its own as a sentence, whilst a subordinate clause is introduced by a conjunction. A phrase is a group of words that does not contain a verb and cannot stand alone as a sentence.
FULL STOP OR PERIOD .
A full stop or period must be used at the end of a sentence, as defined above, unless it is replaced by a question mark of exclamation mark. A common mistake is to use a comma instead of a full stop. In direct speech, a full stop should be placed before the closing speech marks if they mark the end of a sentence.
Full stops or periods are also used after abbreviations, unless the abbreviation ends with the last letter of the word, in which case no period is necessary.
QUESTION MARKS ?
A question mark is placed at the end of a direct question. It comes before the closing speech marks in direct speech, for example:
“Where is the library?” I asked him.
Question marks are not used with indirect (reported) questions, for example:
I asked him where the library was.
This is an indirect question that does not need a question mark.
EXCLAMATION MARK !
An exclamation mark indicates surprise, anger, etc. It could also be used to show that the speaker is raising his/her voice.
“Look out!” shouted Jim.
“Don't be so rude!” cried the old lady.
“I've won a prize!” exclaimed my cousin.
COMMA ,
The comma indicates a pause that is less strong than a paused indicated by a semi-colon or a full stop. If in doubt, it is probably better to omit a comma unless it seems absolutely necessary. In the case of a sentence with a second clause introduced by a conjunction, a comma may be placed before the conjunction if the verb in the subordinate clause has a subject, for example:
I wanted to go abroad but couldn't really afford it. (No comma needed, as the subject of the verb couldn't is I in the previous clause.)
I really wanted to stay in, but the sun was shining and I had no excuse.
Commas are never placed before opening parentheses.
Commas should be used to separate brief items in a list, the last two items being joined by the word and, for example:
I bought apples, oranges, bananas, grapes and pears.
A comma should be placed at the end of direct speech, before the closing speech marks, if the speech is followed by 'she said', 'he asked', and so on. Exceptions would be where a question mark or exclamation mark is required. (See below.)
COLON :
A colon can be used to introduce a list, for example:
When they went on holiday they took very little luggage: a change of clothes; a first-aid kit and some toothpaste.
Another use is to separate two clauses in a sentence, where the second clause explains the first.
The boys went home early: it was too wet to play tennis.
SEMI-COLON ;
A semi-colon marks a stronger pause than a comma, but it is less strong than a full stop, for example:
Don't wait more than half an hour for me; go on ahead.
A semi-colon can separate longer items in a list, for example:
We were asked to bring three dozen paper clips; two balls of string; one small and one large pair of scissors; a tube of glue suitable for sticking paper; a small notepad and a ballpoint pen.
INVERTED COMMAS/SPEECH MARKS/QUOTATION MARKS “
Double speech marks are used to indicate direct speech. They are opened before the first word that is spoken and closed after the final word that is actually spoken. A full stop, comma, question mark or exclamation mark should be placed after the last word but before the closing speech marks. Words and phrases such as 'she said' and 'he asked' are not included within the speech marks. A quotation used within direct speech can be placed in single inverted commas. Indirect or reported speech does not require the use of speech marks. Here are one or two examples:
“Will you remember to pick Robert up on your way home?” she asked.
He replied, “I've never forgotten to pick him up.”
“Who first said, 'Variety is the spice of life'? Answer me that!” he challenged us.
They told us that it was too soon to make a reservation. (This is reported or indirect speech that does not need speech marks.)
In dialogue, start a new line every time there is a new speaker.
APOSTROPHES '
Apostrophes are perhaps the most misused of all punctuation marks. They have two uses: the first is for contraction, indicating missing letters in shortened words; the second is for possession, where there is an element of ownership or belonging. Examples:
The doctor's surgery was empty. (only one doctor)
The dogs' paws were covered in mud. (more than one dog)
The children's books were all over the floor. (The apostrophe is before the s, because the plural form children does not have an s.)
James's house was burgled last night.
The only exception regarding apostrophes for possession is its. It's with an apostrophe means it is or it has. If its means belonging to it, no apostrophe is used, as in this example:
The cat washed its face.
Examples of apostrophes used for contraction are don't, didn't, can't, isn't, shouldn't, I've, you're, they're, it's.
ELLIPSIS ...
Three successive full stops or periods, known as an ellipsis, can be used to indicate an unfinished sentence.
DASH
A pair of dashes can be used to separate a phrase or clause within a sentence that could be omitted and leave a sentence that would still make sense. The separated phrase or clause usually adds extra information to the sentence, for example:
I'll wear my red shoes – the ones I bought in Italy last summer – as long as it doesn't rain.
HYPHEN -
A short hyphen is used between two words to form a compound word. Not all compound words require hyphens, but a dictionary will tell you whether or not one is needed.
Capital letters are not strictly speaking punctuation, but of course every sentence must start with a capital letter. The first word in direct speech must always have a capital letter, even if the spoken words are not at the very beginning of a sentence. Proper nouns should always have a capital letter; these includes names of people and places, days of the week, months of the year, titles of books or films, and so on.