Commas in English

Commas are complicated! Here are a few rules. One general piece of advice: if you have a lot of commas in a sentence, consider dividing it into two (or more) sentences or changing the word order.

Commas as brackets

Commas can be used in place of brackets. Make sure that the sentence isn't too long and that it is clear which part of the sentence is in comma brackets. 

My friend Li, who comes from China, doesn't have any brothers or sisters.

Tip: If the sentence is long with a lot of commas, consider using real brackets, e.g. 

My friend Li (who comes from China) doesn't have any brothers or sisters.

Commas to separate introductions

Commas should be used to separate the introductory element from the rest of the sentence.

In 2014, elections for the European Parliament took place.

In general, you should go to the airport one hour before your flight.

However, ...

For example, ...

This is also true if the words come in the middle of the sentence:

You can, of course, go to the airport two hours before your flight.

I would recommend, however, that you only go one hour before.

Tip: If the sentence is long with a lot of commas, consider putting this information at the end of the sentence, e.g. 

Elections for the European Parliament took place in 2014.

Commas when adverbial or subordinate clauses come first

I call these "flipped sentences". In this kind of sentence, the "because", "as", "due to", "owing to", "although", "if", etc. comes first.

Because animals were not allowed in the student dorms, I had to give away my hamster to a friend.

Although I learnt to play the piano when I was younger, I don't think I would be able to read music now.

Tip: If the sentence is long with a lot of commas, consider flipping the sentence and removing the commas e.g. 

I had to give away my hamster to a friend because animals were not allowed in the student dorms."

Examples of sentences with a lot of commas:

Owing to the one-child policy, my friend Li, who comes from China, doesn't have any brothers or sisters.

Here it would be better to remove one set of commas:

My friend Li, who comes from China, doesn't have any brothers or sisters owing to the one-child policy.

Owing to the one-child policy, my Chinese friend Li doesn't have any brothers or sisters.

Extra advice for native German speakers:

No comma before "that".

which or that

This one is quite difficult to explain. I don't know the rule, but this is how I do it:

If the information after the which/that is essential information, then use "that".

Example with "that"

Where is the chocolate?
What chocolate are you talking about?
The chocolate that I put on my desk this morning.




Note that there is no comma before "that".


 
If the information after the which/that is not essential information, then use "which". A good way to check this is whether you can put the information in brackets.

Example with "which"

I'm putting this chocolate, which I bought while I was on holiday, on the table so everyone can help themselves.
I'm putting this chocolate (which I bought while I was on holiday) on the table so everyone can help themselves.

Note that there is always a comma before "which" .


Incorrect
Some people think "which" just sounds better that "that" and there is no difference. They are wrong. It sound bad if you use which where you mean that.

Be careful with the word "also"

The word "also" should be used very little. 

It should not be used at the start of a sentence, unless it's before a comma.

It should be used (i) before the verb, (ii) after the word "is", or (iii) before words like "because".

Alternatively, it can be used (iv) as an introductory word — followed by a comma — to link the sentence to the sentence before it.

For example:

(i) British people are passionate tea drinkers, but they also drink coffee.

(ii) Tea is the most popular hot drink in the UK, but coffee is also popular.

(iii) Tea is popular in the UK because it tastes good, but also because the British Empire had a large presence in India.

(iv) I don't like ice cream because it's sickly sweet and melts all over my hands. Also, I get brain freeze every time I eat it.

Origin of the confusion

German language texts often put "also" at the start of sentences, particularly before nouns.

e.g. Auch Kaffee wird in Großbritannien getrunken. In English, this would be "coffee is also drunk in Britain".

No comma before "that"

In German, you always put a comma before "dass". 

In English, you don't put a comma before "that"!

Examples:

It was shown that trace elements enhance microbial growth. 

Our results demonstrate that high levels of X cause Y.

I thought that you didn't like tomatoes!

Example of incorrect comma use:

It was shown, that trace elements enhance microbial growth.