Have a look at the sentence below:
I like chocolate, for it is yummy.
Which word class do you think for belongs to?
For many of you for is a coordinating conjunction, because it represents the 'f' in FANBOYS. If you don't know what FANBOYS is, it's acronym that acts as a mnemonic for remembering the list of coordinating conjunctions:
f - for a - and n - nor b - but o - or y - yet s - so
If you google FANBOYS you'll find that many websites use the acronym, including Wikipedia. The set of examples used on many of these websites are often similar. Here's an example:
If you look at the first example sentence in the image, you will see that for in this case means 'because', and for that reason it's actually best regarded as a subordinating conjunction in this case.
So is it wrong to have for as part of the FANBOYS acronym?
That question is not so easy to answer because there is some grammatical indeterminacy in this area of grammar. I'll explain why in a moment, but let's first look at and, but, and or.
And, but and or
These are unproblematic as part of FANBOYS, because they are (almost) always coordinating conjunctions. Recall that coordinating conjunctions can link equal units such as words, phrases and clauses:
[dogs] and [cats]
[rain] or [sunshine]
[cool] but [pleasant]
[the dogs] and [the cats]
[the rain] or [the sunshine]
[very cool] but [mostly pleasant]
[I like cool weather] but [I also like hot climates]
But what about about for?
For is actually rather special, because in some cases it behaves like a coordinating conjunction, but in others like a subordinating conjunction.
Reasons for saying that for is a coordinating conjunction
One reason is that clauses introduced by for cannot be placed at the start of a sentence. In this regard they behave just like clauses preceded by a typical coordinating conjunction such as and or but:
*And I like cakes, I like chocolate. (Recall that the * indicates that a sentence is ungrammatical.)
*But I do like fish, I don't eat meat.
*For it is yummy, I like chocolate.
If for were a subordinating conjunction, we would expect it to behave like typical subordinating conjunctions such as because which can appear at the front of a sentence:
Because it is yummy, I like chocolate.
Another way in which for is not like a subordinating conjunction, but like a coordinating conjunction, is that you can't link clauses introduced by for, whereas this is perfectly possible for two clauses introduced by typical subordinating conjunctions:
I like chocolate, [because it is yummy] and [because it is sweet].
*I like chocolate [for it is yummy] and [for it is sweet].
Reasons for saying that for is a subordinating conjunction
There are also reasons for saying that for is a subordinating conjunction.
First of all, as we saw earlier, when it is followed by a finite clause for is an alternative to because, albeit a rather rare and formal one. The following sentences, in which the subordinate clauses function as Adverbial, mean the same:
I like chocolate, for it is yummy.
I like chocolate, because it is yummy.
Secondly, for must always be followed by a full clause; the subject cannot be left out:
*He liked the conference, for was treated as royalty.
Compare this to a sentence that contains the coordinating conjunction but:
This is a very good idea, but may be impractical.
Here the clause after but lacks a subject.
Thirdly, typical coordinating conjunctions can link more than two items, as in this example:
I went to Paris and I visited the Louvre and I had dinner in a fancy restaurant.
Although stylistically this is not a great sentence, it's perfectly grammatical. (For the true grammar nerds among you, this is an example of polysyndetic coordination.) However, you cannot link more than two clauses that start with for:
*I had the steak and chips for I felt like a big meal for I was hungry.
Apart from finite clauses, for can also introduce nonfinite clauses. (They are nonfinite because they have a to-infinitive inside them.)
I arranged [for Michael to meet me in the park].
[For her to call me at three in the morning] was inconsiderate.
In these cases for clearly has a subordinating function.
So we see that for shares some of the characteristics of coordinating conjunctions and of subordinating conjunctions.
The general point that emerges from this discussion is that it's not always that easy to assign words to one grammatical category or other.
With regard to the example sentences in teaching resources illustrating the use of for as a coordinating conjunction: they are almost always wrong because they instantiate the use of for as a subordinating conjunction.