Conjunctions are one of the 8 parts of speech. We need to talk about conjunctions together with clauses because clauses in sentences can be joined by punctuation (often a comma) or punctuation plus some kind of conjunction. Many grammar errors relate to the misuse of punctuation to join phrases and clauses.
Let's review the difference between a clause and a phrase, also explained on the page on sentences. A phrase is any group of related words. A clause is a group of related words containing a subject and a verb. Incorrectly connecting clauses often leads to run-on sentences.
RULE 1: Use a comma to separate main clauses joined by a coordinating conjunction (and, but, or, for, yet, and so.) Notice that the second clause also has a subject. If the subject is not present, then it's not a clause (it's a phrase), so you don't need a comma.
EXAMPLE: The students were delayed by the heavy rain, but they managed to get back to class on time.
RULE 2: Use a semicolon to separate main clauses not joined by a coordinating conjunction.
EXAMPLE: Sometimes we work on grammar worksheets; other days we discuss our novel studies.
RULE 3: Use a semicolon to separate main clauses joined by conjunctive adverb.
(Conjunctive adverbs include: "also," "consequently," "finally," "furthermore," "hence," "however," "incidentally," "indeed," "instead," "likewise," "meanwhile," "nevertheless," "next," "nonetheless," "otherwise," "still," "then," "therefore," and "thus.") Conjunctive adverbs are adverbs that function as a conjunction (they join clauses and they modify a verb).
Examples of Conjunctive Adverbs
I took a course on grammar; also, I took a course on calculus.
I took a course on grammar; consequently, I write much better now.
I took a course on grammar; nevertheless, I still make mistakes.
I took a course on grammar; finally, I know how to use a semicolon.
I took a course on grammar; furthermore, I wrote a novel.
I took a course on grammar; hence, I am qualified to become an editor.
We went to the mountains to go skiing; however, the rain washed the snow away.
We went to the mountains to go skiing; incidentally, the resort was celebrating its anniversary.
I took a course on grammar; indeed, my teachers noticed the improvement.
I continue to learn more grammar rules; meanwhile, my writing assignments get harder.
I am becoming a skilled writer; nevertheless, I still use an editor.
I continue to proofread my essays; otherwise, I will hand in work with errors.
I look forward to graduating; still, I expect to continue to learn.
He told me he had learned all the grammar rules; then, I expect to see no mistakes in his writing.
English is a difficult language to master; therefore, expect to make mistakes.
I have completed my grammar course; thus, I will write much better.
Note that when the conjunctive adverb comes within the second main clause instead of at the beginning, the clauses still must be separated by a semicolon, and the conjunctive adverb set off by commas.
Example: Canadians spend millions of dollars for road-building; our roads, however, are still full of pot holes.
Subordinating conjunctions are different from conjunctive adverbs. A conjunctive adverb is primarily a transitional word carrying the thought from one main clause to the next. Subordinating conjunctions introduce subordinate clauses. Subordinating conjunctions include: "when," "although," "though," "since," "if," "because," "so that," "as," "after," "in order that", "while" and "unless." Subordinating conjunctions are used to express a logical relationship between clauses. Notice that the subordinate clause may come before or after the main clause.
Examples of subordinating conjunctions
I will be an expert grammarian when I complete the course.
Although I make mistakes now, I expect to be better later.
I am sad to see him go, though I know he will be happy.
I am happy to see her move to Zimbabwe, since she has been annoying me for two years.
If you give me five dollars, I will give you a piece of pie.
Because he studied at Oxford, I trust his grammar knowledge.
I sent her to the grammar course so that she will improve her writing.
As I am a dedicated teacher, I try to explain things as much as I can.
We will go for coffee after we have our meal.
In order that his writing improves, I have told him to take a course.
While we are studying in this course we must do our homework daily.
Unless you pay the tuition, you will not be registered.
RULE 4: Use a colon to separate two main clauses, the second of which amplifies or explains the first.
EXAMPLE: The teacher posted a notice on the board: homework will be collected daily.
(The only other time you use a colon is to introduce a list. Do not use a colon to introduce speech or a quotation.)