"STRUNK" - читать интересную книгу автора (Strunk William)

The early records of the city have disappeared, and the story of its first
years can no longer be reconstructed.

The situation is perilous, but there is still one chance of escape.

Sentences of this type, isolated from their context, may seem to be in need of
rewriting. As they make complete sense when the comma is reached, the second
clause has the appearance of an after-thought. Further, and, is the least
specific of connectives. Used between independent clauses, it indicates only
that a relation exists between them without defining that relation. In the
example above, the relation is that of cause and result. The two sentences
might be rewritten:

As the early records of the city have disappeared, the story of its first
years can no longer be reconstructed.

Although the situation is perilous, there is still one chance of escape.

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.

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 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.

For two-part sentences connected by an adverb, see the next section.

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.