3. Members of a compound sentence may present an alternation or choice between two thoughts. In such a case only one of the members is true; as, “Come down and help us, Rustum, or we lose.”—M. Arnold. If the first proposition here is fulfilled, the second will not be true. If the second should be true, it would be because the first was not fulfilled. Often, as in this example, the second member denotes a consequence of something which is the direct opposite of what is stated in the first member.
Connective word.—(a) The typical connective is the coördinating conjunction or. Sometimes the first member is preceded by either, a correlative of or. It may be remarked here that or is much oftener met between words or phrases than between propositions.
(b) The adverbs else and otherwise when equivalent to or may be used in its stead.
4. The second member of a compound sentence may denote a result of the first, either a direct consequence or a conclusion. In other words, the first member may denote (a) the cause of the second member; as, “They speak English there, so your difficulties are now pretty well over.”—Brontë. (b) The evidence which supports the second member; as, “He was not at church Sunday, hence he must have been ill.” Such sentences suggest the complex sentences containing a clause of real cause or a clause of reason; and owing to the causal relation between the two members, it may seem at first sight that one of them is a dependent proposition. But such is not the case. Each of them is grammatically independent; for both are presented as equally important, and the coördinating conjunction and could be supplied between them in addition to the other conjunction.
Connective.—The typical conjunction is therefore. We also find hence, wherefore, so, so that, so then, consequently and accordingly, all of which are equivalent to therefore.
5. Frequently one member is added to another by way of explanation, giving the author’s reason for making the first statement. It is introduced by for and usually preceded by a semicolon; thus, “Always pay; for first or last you must pay your entire debt.”—Emerson.
It is sometimes difficult to decide whether the second member is independent or dependent, whether for is a coördinating or a subordinating conjunction. The tests for the independent proposition must be these:—Did the author intend the second member to stand equal in importance to the first? Does the second member add something not necessary to the first member or affecting its meaning? Does the second member express a thought not perhaps in the author’s mind when he wrote the first?
Omission of the Conjunction in Compound Sentences.—Often when the relation between two members is perfectly evident, the connective is omitted. This frequently occurs when the members are in the same line of thought; as, “The place to observe nature is where you are; the walk to take today is the walk you took yesterday.”—Burroughs.
The connective is also omitted to avoid repetition when the sentence contains several members, all sustaining the same relation to one another. It should, however, be noticed here that authors differ in their mode of punctuation. One author might combine two sentences into one compound sentence by placing a semicolon between them, whereas another would place a period after each.
The Number of Members in a Compound Sentence.—In illustrating compound sentences we have given those that contain only two members, but it must not be supposed that the number is restricted to two. On the contrary we often find three or more. However, these are not often all of the same rank, and the sentence must first be separated into coördinate members, then if these members are compound they must be separated in like manner, and so on. Take, for instance, the sentence,—“Genius is mainly an affair of energy, and poetry is mainly an affair of genius; therefore a nation whose spirit is characterized by energy may well be eminent in poetry,—and we have Shakespeare.”—M. Arnold. This complex-compound sentence is, first of all, made up of a compound sentence and a complex-compound sentence. The relation between these two members is that of cause and result, the second being a consequence of the first, as is indicated by the conjunction therefore. The first of these members is made up of two simple sentences in the same line of thought, therefore joined by the conjunction and. The second member is made up of a complex sentence and a simple sentence, the second being an addition to the first and therefore joined to it by and.