Weak Conjunctions Between Independent Clauses

Every independent clause can stand alone as a sentence. When you join two of them, the conjunction you pick is a claim about how they relate. 'So' claims cause and result. 'But' claims contrast. 'And' claims only addition, nothing more. The fix for a weak joint is never grammar in the narrow sense, since 'and' is always grammatical between two independent clauses. The fix is meaning: read what the first clause is doing to the second, then choose the word that says that out loud.

This is why a sentence like 'The trail was icy, so we turned back, and it got dark' still sags even though both halves are technically fine. The first joint, 'so', correctly marks cause. The second joint just bolts on another fact with 'and', so the sentence reports three events as if they were an equal, unordered pile, when really 'it got dark' is background, not a third consequence. Stacking clause after clause with 'and' never trips a grammar rule, which is exactly why it is tempting and exactly why it reads as flat.

Subordination gives you a second tool once coordination feels too loose. Turning one clause into a dependent clause, with a word like 'because' or 'when', ranks the two ideas instead of listing them side by side: 'Because the trail was icy, we turned back' tells you immediately which event is the reason and which is the result. This matters for the next skill in the unit too, since a dependent clause that gets left dangling, or wrongly punctuated as if it were independent, creates the fragment and comma problems that follow directly from this same choice.