Or the subordinate clauses might be replaced by phrases:
Owing to the disappearance of the early records of the city, the story of its first years can no longer be reconstructed.
In this perilous situation, there is still one chance of escape.
But a writer may err by making his sentences too uniformly compact and periodic, and an occasional loose sentence prevents the style from becoming too formal and gives the reader a certain relief. Consequently, loose sentences of the type first quoted are common in easy, unstudied writing. But a writer should be careful not to construct too many of his sentences after this pattern (see Rule 14).
Two-part sentences of which the second member is introduced by _as_ (in the sense of _because_), _for_, _or_, _nor_, and _while_ (in the sense of _and at the same time_) likewise require a comma before the conjunction.
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.