The sentence, as written above, contains one form of the sentence error—one of the worst possible mistakes in writing. It is sometimes called the comma fault or the baby blunder. For brevity we shall call it S 1 (sentence error number one). Motor wagons are economical is a principal clause. Department stores of all large cities are acquiring them is also a principal clause. Two such clauses may not stand in the same sentence separated only by a comma. To correct, divide into two sentences; as,
Right: Motor wagons are economical. Department stores of all large cities are acquiring them.
Sometimes the thought in the two principal clauses is closely connected. In that case they may be put into the same sentence, provided they are properly connected or separated. Use a comma plus a coördinate conjunction (as and, or, but) to connect them, or a semicolon (;) to separate them.
Be particularly careful of the conjunctive adverbs so, then, therefore, thus, also, still, otherwise, however, hence, consequently, moreover, nevertheless. When they are used to join the principal clauses of a compound sentence, a comma is not sufficient punctuation between the clauses. A semicolon or a comma and a coördinate conjunction must be used.
Wrong: He had been a good customer, so they were sorry to lose his trade.
Right: He had been a good customer; so they were sorry to lose his trade.
Right: He had been a good customer, and so they were sorry to lose his trade.
S. 2.—The first form of the sentence error (S 1) is made by using too much for one sentence. The second form (S 2) is made by using too little. It consists in writing a subordinate clause or a phrase as a sentence; as,
1. Wrong: I told her I would attend to the matter at my earliest convenience. Probably on my way from work in the evening.
2. Wrong: His doctor advised him to go to Arizona. Which he decided to do.