It is also true that the inventory of punctuation marks is much more limited than the devices we use to signal, say, the completeness or uncertainty of something we say, especially since, by convention, we punctuate only at the end of groups of words. Poets have a better time of it, since they are able to use lines and extra spaces as subtle aids to the emphasis of their sentences. But even poets have to know what rules they are breaking if they are to break them effectively: a knowledge of grammar does not make punctuating easy, but an ignorance of grammar makes it impossible.
Proper punctuation is a matter of recognizing two things:
Once you have decided to use punctuation, it will help if you make a distinction between the two main kinds of punctuation marks.
Thus it is sentence structure that you must consider in taking these decisions.
Periods and the like identify word groups that can stand alone, that is, sentences containing complete predications (or understood substitutes for complete predications). Commas and the like divide sentences into parts in order to facilitate reading. Too many periods produces sentence fragments:
24a While Michael went to the store. Heather studied.
Too few periods will result in run-on sentences or
24b Michael went to the store Heather studied.
24c Michael went to the store, Heather studied.
Replace the period after "store" in 24a with a comma or insert a semi-colon in the same spot in 24b and 24c and you have perfectly acceptable punctuation, the first indicating the presence of an introductory (adverbial) subordinate clause, the second the existence of two coordinate clauses. To avoid this kind of punctuation problem, at least in such straightforward cases, you simply have to be able to recognize coordinate and subordinate clauses.
You have to be able to recognize subjects and predicates to avoid the common error of wrongly separating these fundamental sentence parts. If you rely simply on your sense of the pauses of speech, you may be tempted to write
25 The time that I went to Disneyland, was a real high point in my childhood.
The comma here suggests that the subject and predicate belong only loosely together, whereas they are the inseparable constituents of the the act of predication that forms a sentence. This is not to say that no comma can occur between a subject and a predicate, but such a comma must either be needed to set out a list or it must be part of a pair of commas setting off an insertion. Sentence 5d provides an example of this kind of construction:
Olga, who wants to go to graduate school, studies whenever she has a moment.
The commas around "who wants to go to graduate school" indicate that we are treating this clause as a kind of parenthetical comment, something interesting to know but not the distinguishing feature of our friend Olga. We call such a clause non-restrictive. If we knew two Olgas, of whom one wants to go to graduate school, the other to law school, we couldn't dispense with the clause identifying her aspirations, so we would remove the commas; the resulting sentence
Olga who wants to go to graduate school studies whenever she has a moment.
would indicate to the attentive reader that the clause was restrictive and important to know. But in neither case would we allow a single comma to intervene between subject and verb.