never
Proper nouns
Unless they are proper nouns or begin a sentence, no.
No, you don't. The words "swine" and "influenza" are not proper nouns.
In a list, you would typically capitalize the first word of each item, proper nouns, and the first word after a colon if it introduces a complete sentence.
No, they are not proper nouns.
Yes.
Capitalization is determined by whether a word is a noun or a proper noun. Do not capitalize robin or deer, they are nouns, not proper nouns.
Because they aren't proper nouns.
Yes! Capitalize all proper nouns, including cities.
Yes, you would capitalize Twitter, since it is a proper noun. All proper nouns are capitalized.
It is a proper noun, and proper nouns are always capitalized.
Yes, because "French" is a proper noun, and most of the time you capitalize proper nouns.