Commas ,

The first use of a comma comes after some introductory text in a sentence, which might be a single word, a short phrase or a longer clause.

Using a comma after an introductory item helps to guide readers by showing that the opening is in fact a secondary piece of information, and the main clause in the sentence starts after the comma. This helps reading fluency, even if most readers are not conscious that they are getting these cues as they read.

Where you have a short introductory item in a longer sentence, you might decide to leave out the first comma for rhetorical flow. This will be more common in literary texts. But in practical and workplace documents, it is safer to use the introductory comma and keep the sentences short.

Nothing quite divides the English speaking world like the use of commas in serial lists. These are short lists within a sentence that separate each item with a comma or conjunction. 

Australian style uses commas between the first items, but only a conjunction before the last.  

If the items on a serial list are longer than these, you may need to separate them with semicolons or (preferably) turn them into a vertical list.

Proponents of the Oxford comma argue that without it, it is not clear whether there are 2 items or 3 in the list. But in the examples they tend to cite, ambiguity is created more often by unlikely semantics than punctuation. Apparently they go to parties where ‘the host invited their parents, JFK, and Stalin’ and would not be able to tell that 2 world leaders were not the host's parents without the extra comma.

Australia has got on rather well for decades without the Oxford comma. The ASG recommends not using it unless there is potential for ambiguity in a sentence.

A more clear-cut use of the comma relates to restrictive and non-restrictive elements within a sentence.

Restrictive clauses or phrases are essential to the meaning of a sentence. If you delete them, the sentence will say something different and might not make sense. Don't put commas around these.

Non-restrictive clauses or phrases are the opposite and usually provide extra (optional) detail. You can take them out and it won't affect the sentence's overall meaning. You should use commas around these.

As a clue, look out for the words:

• ‘that’, typically signalling restrictive text (no commas)
• ‘which’, typically signalling non-restrictive text (commas).

Note that where you include a position title, you don't need to add commas if you're using the title as an adjective rather than a noun or noun phrase. 

It will become non-restrictive if you add a determiner such as an article at the front.

Another major point of confusion surrounding commas relates to whether to place them between clauses. This is partly because of the old primary school rule that ‘you can’t put a comma before a conjunction’. You can and in some cases you should.

The placement of commas between clauses depends on whether they are dependent or independent clauses.

Independent clauses can stand alone as sentences. They usually have a subject–verb–object structure. When they are joined to another independent clause by a conjunction, it's good practice to put a comma in front of the conjunction. This helps the reader follow that each clause starts with its own subject.  

Watch out for sentences that join 2 independent clauses with a comma but no conjunction. This is known as a comma splice and is best avoided. If you don't want to use a conjunction, upgrade the comma to a semicolon or break it into 2 sentences.

If you're quoting direct speech within a sentence, Australian style is to use a comma after the verb that introduces it.

However, if you follow the verb with ‘that’ as a conjunction, you should remove the comma.

If you're formatting your quote as a block quote on a new line, use a colon to introduce it instead and leave out the quote marks.