Use quotation marks to set off new terms that you’re introducing:

Once you’ve introduced a new term, it is no longer new to the reader, so if you use it again, do not put quotation marks around it.

When you use a word as a word — rather than using it in the usual way, to refer to something else — you can put quotation marks around it:

The use of “for” in these questions is unusual, but that’s the word that appears in every translation I’ve seen.

Another correct option is to put words that are used as words in italics:

The use of for in these questions is unusual.

Whichever format you choose, follow it consistently throughout your document.

Do not use quotation marks for common expressions or to call attention to ordinary words:

You might consider these questions “unusual,” but they are “part and parcel” of my heritage.