Punctuation helps a reader see how a sentence is grouped. A comma may mark a boundary, set off added information, or separate items in a list. The mark supports an analysis, but the words and their relationships determine what the groups are.
Two ways a relative clause can attach
That stalled is an integrated relative clause inside the noun phrase headed by engine.
The visitors, who complained, waited: the relative clause is supplementary.
start with the relationship
Let the mark direct your attention
What do the commas change?
Without commas, who complained is integrated: it helps identify which visitors waited.
With commas, the same clause is supplementary: added information about visitors already identified.
The words are identical; the commas support different relationships. Without them the clause sits inside the nominal and narrows the reference. With them it stands apart, commenting on a group the noun phrase has already picked out. The punctuation is evidence for that relationship, not the relationship itself.
The comma sits between two coordinate clauses but belongs to neither tree branch. The two clause frames and and establish the coordination.
a mark is not a clause type
The same caution applies to exclamation marks
The diagram analyses how fast she ran as an exclamative clause inside the object slot. The final exclamation mark is outside that clause.
“Stop!” can take the same punctuation without containing an exclamative clause. A comma before and can also occur inside a noun-phrase list. First find the units and their jobs; then let punctuation support the reading you can already explain.
Let punctuation support the relationship you find.
A comma can point to a likely boundary, but the same mark appears in lists, appositives, coordinated clauses, and supplements. Identify the words and their relationships, then use the punctuation to check that reading.
Ten sentences from this lesson, one decision at a time.