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It's a small line but can be important. In this case, it saves a life:
A comma structures a sentence to make the content easy to understand. It is like a small break in the sentence, separating words, phrases, and clauses. Let's have a look at the basic rules.
Use a comma to separate three or more items in a series.
a) We went to the zoo, the beach, and to the town center. (series of words)
b) The teacher went to the school, had a lot of fun with his colleagues, wrote a lot to the board , and went home again. (series of clauses)
Lists of adjectives
If you are using more than two adjectives, commas may be needed to separate them. If the words can be shifted in order and still make sense or be separated by the word “and”, then a comma is needed between the adjectives. If the adjectives only make sense if used in a certain order, or they cannot be separated by the word “and,” then no comma is needed.
My sister is a nice, shy person.
But: I love your dark brown eyes.
Using direct speech, we need to place quotation marks and commas. The main rules are easy:
1. "You are a nice person," she said.
If the spoken words are quoted before telling who has said them, you place the comma in front of the last quotation mark.
2. She said, "You are a nice person."
The introduction is placed before the direct quotation here. You need to place the comma before the quotation marks before the spoken words.
As the spoken sentence is complete, the first word starts with a capital letter.
3. "And you," Tom said, "are a great cook."
If the speech is broken by the information about the speaking person, we need to combine rule 1 and 2.
1. If the directly spoken part takes the form of a question or an exclamation, you need to use the question/exclamation mark instead of the comma:
"Get out!" she shouted.
"Why?" he asked.
Long and complex sentences are difficult to understand. Commas help to understand the meaning.
Complex sentences are a combination of one main clause and one, or more, subordinate clauses.
|Wherever you go,
||you will always find somebody to help you.
| After I finished my school,
|| I studied medicine,
||to help other people.
The commas help you to structure the sentence. If you leave them out, people will have problems to understand the sentence.
1. Relative clauses
a) Defining relative clauses
Subordinate clauses starting with "who", "which", "that", "whom" and, "where" are called relative clauses. If the part of information given by the relative clause is necessary to understand the whole sentence, it is called a defining (or restrictive) relative clause. Defining relative clauses AREN'T separated by commas.
| Those passengers
|| who sit on the left side
|| change with those passengers
|| who sit on the right side.
The information given by the relative clauses are necessary to understand the rest of the sentence (The sentence, "Those passengers change with those passengers, " makes no sense at all.) So it is necessary and is not separated by commas.
b) Non-defining relative clauses
If the piece of information, given by the relative clause, is not important to understand the full sentence, it is called a non-defining (or non-restrictive) relative clause. The commas are used like brackets here and show that this information is just and addition.
||, who is 36 years old,
||likes to fly.
It's not important to know that Mike is 36 years old, so this information could be left out. It is separated by commas.
Quite often commas are used for sentence parts that are not essential for the sentence, but just give more information.
Like in the sentence you have just read: ", but just give more information." is not necessary, because the part "that are not essential for the sentence" explains the same. It's similar to non-defining relative clauses, where the additional information can be left out.
Some more examples of "additional elements":
1. I am a fantastic man and, of course, I'm a good father too.
The "of course" can be left out without changing the meaning of the sentence, so it is put between commas.
2. In his latest book, "Man on the Moon", John writes about the evolution of a human race on the moon.
As we have already got the essential information which book we are talking about (=>his latest book), the title is only additional information which is marked off in commas.
Use a comma before and, but, for, or, nor, so, yet when it connects two complete clauses (independent clauses).
a) He looked around a corner, and then he walked away.
1. He looked around the corner.
2. Then he walked away.
b) I have eaten lunch, but I can’t tell you what I did before that.
1. I have eaten lunch.
2. I can’t tell you what I did before that.
Wrong: I have eaten lunch(,) but can’t tell you what I did before that.
( Both sentences need a subject and a verb to be complete: “But can’t tell you what I did before that” is not a complete sentence – the subject is missing.)
If you use an if clause, then you must place a comma before the main part of the sentence. If the "then" is left out of the sentence, you still need to place the comma.
Let's have a look at the two sentences above:
| If you write an if clause,
|| then you must place a comma before the main part of the sentence.
| If the "then" is left out of the sentence,
|| you still need to place the comma.
But: Don't place a comma if the "If-part" is at the end of the sentence.
|Don't place a comma
||if the "If-part" is at the end of the sentence.
Actually, there should be a long explanation about using a comma after an introductory clause, phrase or word.
These would be boring, complicated and, quite often, confusing. Let's try to simplify the rule (even on the cost of accuracy):
If in spoken language you use a voice stop, a pause, after the beginning part of the sentence, use a comma.
In hard times like these, we must all be careful.
However, he did not agree with her.
"Yes", "No", and "Well" are always followed by a comma if they are used at the beginning of a sentence.
Yes, he has has learned the rules well.
No, he won't have any mistakes in the following exercises.
Well, at least I hope so.