An em-dash is typically used as a stand-in for a comma or parenthesis to separate out phrases—or even just a word—in a sentence for various reasons (e.g. a parenthetical; an ersatz-ellipsis). Examples where an em-dash should be used:
- School is based on the three R’s—reading, writing, and ’rithmetic.
- Against all odds, Pete—the unluckiest man alive—won the lottery.
- I sense something; a presence I've not felt since—
An en-dash is used to connect values in a range or that are related. A good rule is to use it when you're expressing a "to" relationship. Examples where an en-dash should be used:
- in years 1939–1945
- pages 31–32 may be relevant
- New York beat Los Angeles 98–95
- When American English would use an em-dash – following British and Canadian conventions.
A hyphen is used to join words in a compound construction, or separate syllables of a word, like during a line break, or (self-evidently) a hyphenated name.
- pro-American
- cruelty-free eggs
- em-dash
- it's pronounced hos-pi-tal-it-tee
- Olivia Newton-John
The minus sign is distinct from all three of the above.
If you want to use the correct dash or hyphen in Stack Exchange comments, just use the appropriate HTML entity: — for em-dash, – for en-dash, and − for the minus sign. The hyphen is, of course, directly on your keyboard.
Figure dash
The figure dash (‒) is so named because it is the same width as a digit, at least in fonts with digits of equal width. This is true of most fonts, not only monospaced fonts.
The figure dash is used within numbers (e.g. phone number 555‒0199), especially in columns for maintaining alignment. Its meaning is the same as a hyphen, as represented by the hyphen-minus glyph; by contrast, the en dash is more appropriately used to indicate a range of values; the minus sign also has a separate glyph.
The figure dash is often unavailable; in this case, one may use a hyphen-minus instead. In Unicode, the figure dash is U+2012 (decimal 8210). HTML authors must use the numeric forms ‒ or ‒ to type it unless the file is in Unicode; there is no equivalent character entity.
Formally, hyphens are for joining terms, and en dashes are for ranges and distinctions. En dashes have a secondary application joining terms that are already hyphenated or contain spaces, but that doesn't apply here. In this situation I would use an en dash or a slash. Of course, if you do write it with a hyphen, nobody is going to be confused.
Technically, cost–benefit analysis can be interpreted as either [cost–benefit] analysis—an analysis of costs versus benefits—or cost–[benefit analysis]—costs versus an analysis of benefits. Luckily, the latter doesn't make any sense and would really only arise from deliberate pedantic misinterpretation.
Best Answer
Chris' comment is actually the answer, but I can't format things well in comments
Oxford Advanced Learner's Dictionary 8th edition (app edition), which includes contents from OALD 2010 and Oxford Learner's Pocket Thesaurus 2010 does not have an entry that says "neverending" (compound) that your teacher suggests, what it does have is "never-ending" (your version).
As of this writing (typing?) Oxford's online resource does not have "neverending", what it does have is "never-ending"
The same stance is also taken by UK Advanced Cryptics Dictionary (this dictionary is used by Opera, the browser I'm using). Opera highlights "neverending" as wrong, suggesting corrections as "never ending" or "never-ending"
The free dictionary also says the same: if you try to search for "neverending", it will suggest "never-ending" or "never ending" instead
While technically you can join two words into one like "neverending", I'd say that your usage is much more common and recognized. Perhaps you should speak about this to your teacher.