Are both of the following sentences correct?
a: You can call me, if you need me. OR
b: You can call if you need me.
Note that in a:, the comma is placed before the "if" and is not present in case b.
From this link, I gather that it isn't necessary because it's a short sentence: Is it mandatory to use a comma before a coordinating conjunction uniting the two independent clauses in a compound sentence?
But can someone point out an "official" source on this usage?
The question that you brought up wouldn't address your question because "if" is not one of the coordinating conjunctions, but it is a subordinating conjunction. The words that introduce each are:
Coordinating Conjunctions: and, but, or, yet, for, nor, so (a helpful acronym is FANBOYS)
Subordinating Conjunctions: after, although, as, as if, as long as, as though, because, before, even if, even though, if, if only, in order that, now that, once, rather than, since, so that, than, that, though, till, unless, until, when, whenever, where, whereas, wherever, while
The definition of a subordinating conjunction is:
Subordinating Conjunction (sometimes called a dependent word or subordinator) comes at the beginning of a Subordinate (or Dependent) Clause and establishes the relationship between the dependent clause and the rest of the sentence. It also turns the clause into something that depends on the rest of the sentence for its meaning.
He took to the stage as though he had been preparing for this moment all his life.
Because he loved acting, he refused to give up his dream of being in the movies.
Unless we act now, all is lost.
This article from Purdue on coordinating and subordinating conjunctions explains the usage of the subordinating conjunction as follows:
Notice that when the subordinate clause comes at the beginning, it’s necessary to insert a comma.
From this, the correct punctuation of "You can call me if you need me" is:
You can call me if you need me.
If you were to move the subordinating conjunction to the beginning, however, you would need the comma as follows:
If you need me, you can call me.
As in many other aspects of the English language this is an area where British and American usage reveal differences. The trend in British English writing is towards lighter internal sentence punctuation. (The Oxford comma is a case in point.) It is common to omit the comma even when the dependent/subordinate clause begins the sentence.
Of course, there are times when comma omission will result in momentary ambiguity and is to be avoided:
As to official rules on comma use to separate clauses, there are none (unlike in German for example where the comma is mandatory between dependent and independent clauses regardless of where the clauses are placed.)
The previous sentence is an example of the light punctuation that I as a British writer prefer but which would seem careless to some American readers.
Both are correct. The implied nuances (perhaps rather overstated) are (a) "You can call me, if you need me [or even if you just want a chat]" and (b) "You can call me if you need me [but please don't bother me for some trivial reason]".
