The dash as a mark of punctuation should be little used by beginners in composition writing.

XXVIII. The dash should be used to show an interruption - sometimes sudden and abrupt. It suspends the construction in order to change it, or to give an unexpected turn to the end of the sentence.

But there was no unusual sound - nothing but the low wash of the ripple and the croaking of the crows in the wood.

Here are these beautiful fields - I will show you the way through them.

XXIX. A dash, usually following a comma, often introduces an explanation.

Uncas enjoyed his victory, but was content with merely exhibiting his triumph by a quiet smile, - an emblem of scorn which belongs to all time and to every nation.

XXX. Dashes set of a parenthetical expression which is too closely connected with the rest of the sentence to be inclosed in marks of parenthesis, and yet needs to be distinctly separated from the rest of the sentence. Commas should precede the dashes, if commas would be necessary provided the words between the dashes were omitted.

Thomas Hughes, too, is gone, - Tom Hughes would still seem the more accustomed name, - one of the many men who illustrate the somewhat painful truth that the heights of philanthropy and self-devotion do not yield so sure a fame as a spark of genius.