Only a complete sentence (including a verb) gets a period.
(Two complete sentences can also be joined by a semicolon, i.e. ; .)
Commas are used to set off phrases from the main body of the sentence.
(Their placement is determined by sentence structure, NOT by pauses in speech rhythms.
If any teachers or instructors ever told you that, take a moment to expunge it from your memory forever, and say a prayer for them. They need it.)
Remember possessive apostrophes, e.g. Hegel's book, not Hegels book.
With plurals, put the apostrophe after the final s, e.g. : the citizens' votes.
(Treat biblical and ancient Greek names ending in s as if they were plurals, e.g. Sophocles' play. Modern names ending in s don't get this special treatment. They just get the usual, as in James's starship, the same as Jim's starship. )
However, the possessive "its" as in "its color is startling" does not have an apostrophe.
It doesn't need one, because it is equivalent to "his, her, their.."
The possessive "its" can thus be differentiated from the contraction of "it is," which does have an apostrophe.
Thus, "It's important always to include its apostrophe in any contraction."
Remember the apostrophes in contractions, e.g. doesn't, isn't, it'll, it's (as in "it's raining").
Not certain whether you have a contraction or not? See if you can separate it out, as in "it is raining." Easy!
Now, rules for quoting:
Quotation marks should, in the U.S., be put outside periods and commas, even when the periods or commas are not in the original. E.g. "Labyrinths." (Not "Labyrinths".) (The rules are different in the United Kingdom, that is, England, Scotland, etc..)
However, don't put quotation marks outside semicolons and colons.
Do use quote marks around any phrase or longer passage taken directly from any other author, to avoid plagiarism. Give explicit credit to your source.
Careers have been ruined by people neglecting to do this, but it's only common courtesy to cite your author.
If the quote fills less than three full lines of your text (some disciplines use a two- or four-line threshold; it doesn't really matter to most of us),
then simply put quote marks around the quote, and include the citation in parentheses after the last quote mark, but before the period.
. . . . as we read in Berger, "being envied is a solitary form of reassurance" (Berger, p. 133). Here Berger reflects the analysis of . . .
If, however, the quote is three full lines or more of your text (or two or four, whatever),then it must be treated as a block quote. Block quotes have special rules.
Omit the quotation marks.
Instead, set the quote off from your surrounding text by indenting the whole quote on the left margin.
(Also, the citation at the end is placed after the period, unlike usual quotes.)
. . . lada lada lada . . .
Schopenhauer elucidates this point, also:
[M]atter is absolutely nothing but causality, as anyone sees immediately the moment he reflects on it. Thus its being is its acting . . . only as something acting does it fill space and time . . . (Schopenhauer, The World as Will and Representation, vol. I, pp. 8-9)
Surely Schopenhauer was aware of his debt to Spinoza when he wrote these lines. In fact, . . . . lada lada lada . . .
If you omit any words from a quote, let your reader know that by putting ellipses where the omitted words were. (Ellipses: i.e. 3 dots with a period between each.)
(E.g. the preceding quote, in which I removed one sentence and then ended the quote mid-sentence.
Or e.g. "The everlasting . . . coming to be" )
If you alter any words or letters from a quote, even changing capital letters to lower case and vice versa,
alert your reader to your change by putting brackets around the changed words or letters.
(E.g. Nietzsche remarks that "[t]he everlasting cycle of coming-to-be" )
All of this may not look simple to begin with, but after you write just a few papers taking note of these rules, you'll know them for life. They really do help clarity.