If the second member is introduced by an adverb, a semicolon, not a comma, is required (see Rule 5). The connectives so and yet may be used either as adverbs or as conjunctions, accordingly as the second clause is felt to be co-ordinate or subordinate; consequently either mark of punctuation may be justified. But these uses of so (equivalent to accordingly or to so that) are somewhat colloquial and should, as a rule, be avoided in writing. A simple correction, usually serviceable, is to omit the word so and begin the first clause with as or since:
I had never been in the place before; so I had difficulty in finding my way about. As I had never been in the place before, I had difficulty in finding my way about.
If a dependent clause, or an introductory phrase requiring to be set off by a comma, precedes the second independent clause, no comma is needed after the conjunction.
The situation is perilous, but if we are prepared to act promptly, there is still one chance of escape.
When the subject is the same for both clauses and is expressed only once, a comma is required if the connective is but. If the connective is and, the comma should be omitted if the relation between the two statements is close or immediate.
I have heard his arguments, but am still unconvinced.
He has had several years' experience and is thoroughly competent.
5. Do not join independent clauses by a comma.
If two or more clauses, grammatically complete and not joined by a conjunction, are to form a single compound sentence, the proper mark of punctuation is a semicolon.
Stevenson's romances are entertaining; they are full of exciting adventures.
It is nearly half past five; we cannot reach town before dark.
It is of course equally correct to write the above as two sentences each, replacing the semicolons by periods.
Stevenson's romances are entertaining. They are full of exciting adventures.
It is nearly half past five. We cannot reach town before dark.
If a conjunction is inserted the proper mark is a comma (Rule 4).
Stevenson's romances are entertaining, for they are full of exciting adventures.
It is nearly half past five, and we cannot reach town before dark.
A comparison of the three forms given above will show clearly the advantage of the first. It is, at least in the examples given, better than the second form, because it suggests the close relationship between the two statements in a way that the second does not attempt, and better than the third, because briefer and therefore more forcible. Indeed it may be said that this simple method of indicating relationship between statements is