The correct form for quoting a single line from poem?

The correct form for quoting a single line of poetry is to enclose it in quotation marks. If the line is integrated into your sentence, you don't need any additional punctuation. However, if it stands alone as a complete sentence, it's usually punctuated as a complete sentence within the quotation marks.

Examples:

* Integrated into a sentence: Frost famously wrote that "the woods are lovely, dark and deep."

* Standalone sentence: The poem begins with the haunting line: "The fog comes / on little cat feet." (Note the slash indicating a line break; this is generally preferred for poetry that is visually formatted on the page.)

Remember to always cite the poem and author appropriately after the quotation, usually in a parenthetical citation or footnote.