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The Punctuation and Logic of Compound Main Clauses 
Lassie, Come Home, by Eric Knight
Analysis Key

1. Animals are creatures (PN) {of habit} | -- but new habits can be formed (P). |

The dash here suggests a pause, perhaps an afterthought. The "but" suggests an exception to the idea in the first clause. 
2. But {behind him} the cottage door opened | and his mother's voice spoke. |
The "and" (without a comma) connects two things that happened.
3. Then, [Adv. (space) to heard" when the boy was nearer,] they heard his cry

(DO): | "She's come back! | She's come back!" |

The two clauses that follow the colon are the (same) exact words of his "cry."
4. Rowlie tried to teach her to swing [#1] along {under the wagon} {behind the rear 

axle}, [Adv. to "swing" as a well-trained Dalmatian carriage dog would have done 

{in the days} {of traps and phaetons}]; | but Lassie would have none (DO) {of it}. |

The "but" clause explains Lassie's taking "exception" to Rowlie's plans.
5. [Adv. (place/time) to "will answer and give" Where the cold-blood horse will quit 

and give no more (DO)], the thoroughbred will answer and give another 

burst (DO) {of speed} gallantly, [Adv. (condition) to "will answer and give" even if 

he is spending the last ounce (DO) {of life strength}]; | [Adv. (place/time) to "will 

stand" where the mongrel dog will whine and slink away], the pure-bred will 

still stand {with uncomplaining fearlessness}. |

The semicolon separates two different examples of the difference between pure-bred and mongrel. If the semicolon were a comma, readers might assume that the following "where" clause might chunk back, for example, to ""is spending." The semicolon, on the other hand, is more likely to close the main clause.  [Note the parallel "Where" clauses.]
6. And so Lassie, [Adv. to "treated" although *she was* [#2] now much warier

(PA)], treated these men (DO) [Adv. (manner) to "treated" as she had treated

those other men (DO) {in her own village} [#3]]: | she accepted them (DO), but 

responded {to none} {of them}, nor went [Adv. (place) to "went" where they 

could touch her (DO)], nor answered any (DO) {of their commands}. |

The second main clause, after the colon, gives specific details of how she "had treated."
7. And I got mad (PA) {at him}, [Adv. (cause) to "got mad" [#4] for I don't

owe him (IO) a penny (DO), Duke [#5] or no Duke [#5]], | and I said [DO

[Adv. (condition) to "would see" if she got away again], he'd not see her (DO) 

no more], | and he said [DO [Adv. (condition) to "was" if she ever got away

again] I was welcome (PA) {to her}], [DO but he'd see [DO she didn't]]. |

The three main clauses are joined by ", and" because they express three things that happened.
8. There  [#6] were two opposing forces (PN) struggling  [#6] {in Lassie} | -- one 

*was* to keep [#7] away {from men}; | the other *was* to defend her home [#7]. |

The dash introduces the specific two "forces." The semicolon separates the two different forces.

1. "Her" is simultaneously the indirect object of the infinitive "to teach" and  the subject of the infinitive "to swing" The "to swing" phrase functions as the direct object of "to teach," which functions as the direct object of "tried."
2. This is a semi-reduced subordinate clause. For more on this, see KISS Level 3.2.1 - Semi-Reduced and Other Ellipsed Clauses.
3. Alternatively, "in her own village" can be explained as an adverb (Where?) to "had treated."
4. For more on this see KISS Level 3.2.2 - "So" and "For" as Conjunctions.
5. "Duke" is an appositive to "him."
6. Alternatively, "there" can be described either as an adverb or as an expletive. See KISS Level 2.1.3 - Expletives (Optional). Doing either of those makes "forces" the subject. If "forces" is explained as a predicate noun, then "struggling" is a verbal (a gerundive) that modifies "forces." If "forces" is considered the subject, then "struggling" can be explained as part of the finite verb "were struggling."
7.  If we consider "was" to be ellipsed, then "to keep" and "to defend" are infinitives that function as predicate nouns. ("Home" is the direct object of "to defend.")  Alternatively, the whole sentence can be described as one main clause. From that perspective, "one" and "the other" are appositives to "forces," and the two infinitives function as adjectives to their respective nouns.