Discussion Groups / English Usage / January 2004

There is no fixed rule, and the one the teacher applied is not even
a very good stab at one.  Some commas are structural, and some
indicate pauses, but there are all sorts of ways to approach their
use, and frequently it doesn't matter in a particular context
whether a comma appears or not. (You can go back into that last
sentence and put commas around "in a particular context."  But you
don't have to.)

I'd put both your examples in the "doesn't matter" category, though
other people may differ.  It is even possible that a person
encountering these two sentences not as a pair but with some gap
between encounters might put a comma in one and not the other.  And
there is no way to systematically differentiate the one from the
other where comma use is concerned.  Treat each as you please.

English teachers have a tendency to know so many rules about English
that they know many rules that don't actually exist, or are just
plain false.  This one needs to loosen up.

In the bad old days, I lived with rule-laden English courses.  In high
school we used Hodges & Whitten's _Harbrace College Handbook_, 5th ed.
(1962).  This is what it had to say:

           Introductory Clauses and Phrases
12b
An adverb clause (or a long phrase) preceding the main clause is usually
followed by a comma.

Introductory clauses:
PATTERN: ADVERB CLAUSE, MAIN CLAUSE.

EXAMPLES
    Whenever I tried to put chains on a tire, the car would maliciously
     wrap them around a rear axle. -JAMES THURBER

    If any college man will work intelligently, I guarantee his success.
     -HARDIN CRAIG

Introductory phrases:

EXAMPLES
    /In two years of acting in cowboy films/, W. S. Hart earned
     $900,000.  -H. A. OVERSTREET

    /At the critical moments in this sad history/, there have been men
     worth listening to who warned the people against their mistakes.
     -WALTER LIPPMANN

    Introductory phrases containing a gerund, a participle, or an
infinitive, even though short, must often be followed by a comma to
prevent misreading.

    /Before leaving/, the soldiers demolished the fort.
    /Because of his effort to escape/, his punishment was increased.

    Short introductory prepositional phrases, except when they are
distinctly parenthetical (as in such transitional phrases as /in fact/,
/on the other hand/, /for example/), are seldom followed by commas.

    /At ninety/ she was still active.
    /During the night/ he heard many noises.
    /In fact/, I hope to leave tomorrow. [Transitional phrase]
    /For example/, most boys enjoy fishing.

    Many writers omit the comma after short introductory clauses, and
sometimes after longer ones, when the omission does not make for
difficult reading. In the following sentences the commas may be used or
omitted at the option of the writer:

    If we leave(,) he will be offended.
    When he comes to the end of the lane(,) he should turn to the
     left. [When the subject of the introductory clause is repeated in
     the main clause, the comma is usually unnecessary.]

Note: When the adverb clause /follows/ the main clause, the comma is
usually omitted.

PATTERN: MAIN CLAUSE   ADVERB CLAUSE.

EXAMPLE: I waited there until he returned.

   Such adverb clauses, however, are set off by a comma if they are
parenthetical or loosely connected with the rest of the sentence,
especially if the subordinating conjunction seems equivalent to a
co-ordinating conjunction (or if a distinct pause is required in the
reading).

    Henry is now in good health, although he has been an invalid most of
     his life. [Although is equivalent to but.]

    With children and young people his magic never fails, whether he is
     doing bottle tricks for three-year-olds or counseling teen-agers
     about courses or careers. -ALICE KIMBALL SMITH

  -------------
From the same time period, Perrin & Smith _The Perrin-Smith Handbook of
Current English_ 2nd ed. (1962) gives this table (see the optional rules
#2 & #3)

                COMMAS

Commas are used:

1. Between coordinate clauses (§12.1)
   a) When they are joined by /but/ or /for/
   b) When they are joined by /and/, /or/, /nor/, /yet/ between long
      clauses
2. Between main and subordinate clauses (§12.2)
   a) When the subordinate clause precedes the main clause
   b) When a loosely related subordinate clause follows the main clause

3. Between main clauses and long modifying phrases (§12.3)
   a) When the phrase precedes the main clause
   b) When a loosely related phrase follows the main clause

4. To set off nonrestrictive modifiers (§12.4): /two/ commas are used
   when the modifier occurs within the sentence

5. To set off interrupting elements (§12.5): /two/ commas are used when
   the construction occurs within the sentence

6. To separate coordinate items in a list or series (§12.6)
   a) In lists without conjunctions
   b) In lists with conjunctions

7. To separate for clarity (§12.7)

8. In conventional places (§12.8)
   a) In numbers
   b) In dates
   c) In addresses
   d) With titles and degrees
   e) In correspondence
   f) With phrases identifying direct quotations

Commas are usually optional:

1. Between short coordinate clauses joined by /and/, /or/, /nor/, /yet/

2. After short, closely related subordinate clauses preceding the main
   clause

3. After short, closely related modifying phrases preceding the main
   clause

4. Around short interrupting elements, depending on the emphasis desired

5. Before and joining the last item in a series

6. Between month and year in dates

Yes.  "In 1066" does not add to the necessary information, so it
is a parenthetical phrase.  The one and only Norman invasion occurred
in 1066.

"At 2:30 today" is required information, if you intend to see the
film.

GFH