Third grade teacher here. I plan to teach students to distinguish between simple, compound and complex sentences — but only if I can demonstrate a clear and meaningful difference between the latter two!

Complex or Compound? Each sentence in the following pairs is similar to its partner.

  1. All dogs are mammals, so huskies are mammals.
  2. All dogs are mammals, therefore huskies are mammals.
  3. I come bearing soup, for Kate is sick.
  4. I come bearing soup, because Kate is sick.

Are the sentences above complex or compound?

First, we need to decide whether each italicized clause is independent or dependent. To me, they all look dependent. If that is the case, there's not much need to teach about compound sentences. Is there a meaningful difference between the sentences in each pair that I am missing? (Are the FANBOYS as special as they'd like to think?)

  • I feel for you and it seems blindingly obvious you shouldn't have to even think about that. Teacher's resources and lesson plans should be readily available and clearly indexed, allowing you to teach instead of planning to teach. Sorry to go off topic and it shouldn't matter what jurisdiction you're in or what stage you're teaching; you shouldn't need to be worried about this… Every educational administrator who doesn't understand that should be tasked not with checking whether teachers have planned their lessons according to the latest bean-count, but with planning lessons – Robbie Goodwin Dec 1 '17 at 19:30

It depends on your teaching purpose. Notwithstanding Pitarou's comments and link about FANBOYS, it may well be appropriate for third graders to learn that sentences whose two clauses are connected by one of the FANBOYS (coordinating conjunctions) are called compound sentences. Sentences whose two clauses are joined by one of the other (subordinating) conjunctions are called complex sentences.

Hence in your list sentences 1 and 3 are compound, and sentence 4 is complex. Sentence 2 is somewhat problematic, since therefore is an adverb and some strict grammarians would object to it being used to join clauses thereby producing a run-on or comma splice.

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    In other words, it's important for students to learn how to recognise a dependent and an independent clause. Unfortunately, FANBOYS is not a reliable guide in this matter. Is there a helpful mnemonic to remember which is compound and which is complex, because I always get those two confused. – Pitarou Jun 6 '13 at 3:03
  • @Pitarou. FANBOYS can be used to identify what traditional grammar calls compound and complex sentences, if this is what the teacher wants her grade 3 students to be able to do as an introduction to English syntax (although I personally would stick to the unproblematic coordinators and, but, or, and a few common subordinators). I agree with you, however, that this in itself is of much less importance than understanding that you cannot string independent clauses together without a conjunction (run-on) or write stand-alone dependent clauses (fragment). .... – Shoe Jun 6 '13 at 4:58
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    A useful indicator of coordinators/subordinators is that you can follow a coordinator with a subordinator but the reverse is not possible. For example, you can say but when or so because but not when but or because so. – Shoe Jun 6 '13 at 4:59
  • I can string certain independent clauses together without a conjunction (run-on) or write stand-alone dependent clauses (fragment). And so can many other native English people. The acceptability of this (with certain provisos) has been discussed elsewhere on ELU. – Edwin Ashworth Sep 19 '16 at 19:00

You are quite reasonably worried about edge cases.  Before we look at them, let's look at some much simpler cases.

I like spaghetti and Bob enjoys macaroni.

This is very clearly a compound sentence.  "I like spaghetti" is one clause, and "Bob enjoys macaroni" is another.  Neither clause does any work to or in or for the other.  They are independent, coordinate clauses. 

I like spaghetti when I'm feeling sad.

This is very clearly a complex sentence.  "When I'm feeling sad" is a complete clause with its own subject and predicate.  This clause is inside the matrix clause and it does work to the verb "like".  In this case, it does the same kind of work that an ordinary adverb can do:  "I often like spaghetti."

When I was in grade school and high school, FANBOYS wasn't a thing -- at least, it wasn't a thing that reached me.  The main coordinating conjunctions were "and", "but" and "or".  Occasionally "nor" would pop up without a matching "neither".

The nice thing about this small, tight group is that they can coordinate, oh, nearly anything.  Even simple nouns and pronouns can be coordinated: 

Bob and I like pasta.  -->  Bob likes pasta and I like pasta, too. 

We eat spaghetti or macaroni every Tuesday.  -->  Every Tuesday, we eat spaghetti or we eat macaroni. 

Nobody but Bob cooks pasta in a microwave.  -->  Nobody else cooks pasta in the microwave, but Bob does.

Coordination works the same regardless of the scale.  I can't think of any examples where "for", "so" or "yet" coordinate a pair of nouns or pronouns.  Until I see a good example of something like that, I can't consider them to be coordinating conjunctions.


With that in mind, let's examine your sentences: 

All dogs are mammals, so huskies are mammals.
All dogs are mammals, therefore huskies are mammals.

I don't see either coordination or subordination here.  I see a pair of run-on sentences.  As I interpret them, "so" is an adverb with a meaning similar to "in that fashion" or "as a consequence", and "therefore" is an adverb with a meaning similar to "for that reason" or "as a consequence".  Back in my day, such comma splices were simply considered as errors.

All dogs are mammals.  So, huskies are mammals. 
All dogs are mammals.  Therefore, huskies are mammals. 


I come bearing soup, for Kate is sick.
I come bearing soup because Kate is sick.

In this pair, I do see subordination.  The subordinate clauses do the same job that an adverb would do.  Specifically, the clauses act as adverbs of reason or purpose.


If I were you, I'd ignore FANBOYS entirely.  I'd still teach coordination and subordination.  I'd teach coordination first, from words (Bob and I) through phrases (over the river and through the woods) to full clauses.  I'd then teach subordination by showing how an entire clause can do the same sort of job inside another clause that a single word can do.  The point of having subordinate clauses is making them do the work of nouns, adjective and adverbs.

And there's your meaningful difference.  Coordination means two things working together to do the same job.  Subordination means one thing doing a job for another thing. 

  • I'm a little put off by these examples: "I like spaghetti, and Bob enjoys macaroni," "I like spaghetti when I'm feeling sad." Because it is specifically the conjunction that establishes the relationship between the clauses. You could also say, "I like spaghetti, although Bob enjoys macaroni." Or, "I like spaghetti, but I'm feeling sad." The clauses themselves have little to do with whether one is working for the other or not; that comes only from the conjunction. – Chuckk Hubbard Apr 8 '18 at 23:44

Your intuitions are correct: FANBOYS is not all its cracked up to be.

I don't claim any special authority on the matter, but your examples all look to me like they are functioning as independent clauses.

Here's the first part of an article in which a linguist explodes the myth of FANBOYS. Unfortunately, the author was recently obliged to remove the latter half from his blog when he published it in a journal: suffice to say that he demonstrates that FANBOYS is arbitrary, unreliable, and unhelpful.


In my book, sentences 1 - 3 are compound, though sentence two is punctuated incorrectly. "Therefore" is a conjunctive adverb that requires a semi-colon before it and a comma after it. But conjunctive adverbs, like FANBOYS, are both ways of linking independent clauses, and, therefore, we call them compound sentences.

Your last sentence uses the subordinating conjunction "because." With the use of "because," the second clause becomes a dependent one. It can't stand alone: "Because Kate is sick" is an incomplete thought. That's why this is a complex sentence.

Also note that you will never use a comma before a because clause (unless you have some quirky interrupter right before it). In fact, you will rarely use a comma before any dependent word when the dependent clause follows the independent one.

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