Learn English – What’s the difference between double quotes and single quotes when paraphrasing

This is simply a question of style. Wikipedia has a huge article on the subject. The passages most relevant to your question are:

Quotations and speech
Single or double quotation marks denote either speech or a quotation. Neither style—single or double—is an absolute rule, though double quotation marks are preferred in the United States, and both single and double quotation marks are used in the United Kingdom. A publisher’s or author’s style may take precedence over national general preferences. The important rule is that the style of opening and closing quotation marks must be matched[.]

[...]

Use–mention distinction
Either quotation marks or italic type can emphasize that an instance of a word refers to the word itself rather than its associated concept. [...] A three-way distinction is occasionally made between normal use of a word (no quotation marks), referring to the concept behind the word (single quotation marks), and the word itself (double quotation marks). [...] In common usage, there may be a distinction between the single and double quotation marks in this context; often, single quotation marks are used to embrace single characters, while double quotation marks enclose whole words or phrases[.]

Emphasis mine. Read the entire article for further insight.

Haha, I don't know what their deal is. After a salutation, use a comma for a personal letter, a colon for a business letter, and either for an email. Dashes are not acceptable.

For example

Dear Mr. Lawrence:

¶ This is an example of a business salutation . . .

Dear Sally,

¶ This is an example of a personal salutation . . .

Dear Kane (,/:)

¶ This is an example of punctuation options in an email salutation . . .