This section is from the book "Lessons In English", by Chestine Gowdy, Lora M. Dexheimer. Also available from Amazon: Lessons in English.
A Subordinate Conjunction is a word that joins an adverbial clause to some part of the sentence, but has no use in the clause.
Subordinate conjunctions are like conjunctive pronouns and conjunctive adverbs in joining a clause to « some part of the sentence. They differ from these words in having no use in the clause. They are merely connectives. They differ also in being used to connect adverbial clauses only. Conjunctive adverbs are used to connect either adjective or adverbial clauses (§ 249), and conjunctive pronouns are used with adjective clauses only (§ 142).
Illustration: In the sentence She studies where the light is good, where is not only a connective but an adverb of place in the clause. We can substitute the adverb there for it But in the sentence She studies because she wants to succeed, because is a connective only. No adverb could be substituted for it in the clause. It is a subordinate conjunction.
Describe each clause. Decide which four connectives are conjunctive adverbs and which words are subordinate conjunctions. If a word is a conjunctive adverb, you can substitute for it a simple adverb or an adverbial phrase (Exercise 310 and § 249). This will express the same idea within the clause that the clause does in the sentence.
1. She stopped when I spoke.
2. We eat that we may live.
3. Plant pansies where the soil is rich.
4. She talks as her mother does.
5. We came because we wanted to see you.
6. This wheat is injured by rust, for I have examined it.
7. She is as happy as a lark.
8. Our crops are poor because we have had so little rain.
Look up the meaning of the words coordinate and subordinate. Write a paragraph showing why the two kinds of conjunctions are named as they are.
Analyze the sentences of each passage. Explain the punctuation of each sentence. Make lists of the coordinate conjunctions, the subordinate conjunctions, the conjunctive pronouns, the conjunctive adverbs, and the words that merely introduce noun clauses.
1. Protein makes bone, muscle, and framework for the growing body. Corn is especially short in protein; and to balance the corn ration, we need a feed which is long in protein. Alfalfa is richer in protein than any other farm crop used as feed.
2. When the people discovered that I was quiet, they discharged no more arrows; but by the noise that I heard, I knew that their number increased.
3. I attempted to rise; but I was not able to stir, for, as I happened to lie on my back, my arms and legs were strongly fastened on each side to the ground, and my hair, which was long and thick, was tied down in the same manner.
4. Our country was small when Washington was president. The people lived on the seaboard. The towns and cities were not actually very far apart; but the means of travel were so poor, and the time consumed in going even fifty miles was so great, that the country was practically immense in extent. Now we step into a beautifully fitted car, heated by steam, lighted by electricity, richly carpeted, and provided with the most comfortable seats and beds, and are whirled across the continent from Philadelphia to San Francisco in less time than it took Washington to go from New York to Boston.
 
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