I wonder if there's a rule for which form of "hair" should I use, For example:
Sie hat dunkle Haare.
I'd use the singular form there, as in the English equivalent:
She's got dark hair.
Of course there is a difference in using singular or plural form of "Haar". All dictionaries are consistent in that using "Haar" in singular means all of the scalp hair of a human being. And they all say that it ought to be used in singular in that meaning:
2 ohne Plural: Gesamtheit der Kopfhaare
Charakteristische Wortkombinationen: blondes, braunes, dunkelblondes, dunkelbraunes, fettiges, gefärbtes, gekämmtes, gepflegtes, geschnittenes, gewaschenes, hellbraunes, kurzes, langes, lockiges, mittellanges, schulterlanges, schwarzes, ungekämmtes, ungeschnittenes, ungepflegtes, ungewaschenes Haar
2.a. die Gesamtheit der Haare auf dem Kopf des Menschen; das Kopfhaar
Grammatik
ohne PluralBeispiele
- sie hat schönes, blondes, rotes, braunes, schwarzes, helles, dunkles, lockiges, krauses, glattes, volles Haar
- das Haar lang, kurz, offen, [in der Mitte] gescheitelt tragen
2. Haar, Haare ♦ Gesamtheit von 1, alle Haare, besonders die des menschlichen Kopfes
- er, sie hat blondes, (kastanien)braunes, brünettes, schwarzes, rotes, graues, graumeliertes, (schnee)weißes H.
- dickes, volles, schweres, starkes, hartes, dünnes, feines, weiches, schütteres, langes, kurzes H.
- glattes, krauses, lockiges, gewelltes, gelocktes, glänzendes, seidiges, stumpfes, starres H.
- strähniges, fettiges, trockenes, zottliges, zerzaustes, struppiges, schmutziges H. schönes, gepflegtes H.
- Mit fliegendem Haar und erhitzten Gesichtern liefen gerade die vorbei, die es noch nicht gewohnt waren, zu spät zu kommen — Böll Haus 207
- abgeschnittenes H. wird aufgekauft
- eine Puppe mit echtem H.
- sie trägt ihr H. lang, kurz, offen, in einem Knoten, (in der Mitte) gescheitelt
- das Haar war zu einem griechischen Knoten geschlungen — Musil Mann 95
- (jmdm.) das H., die Haare kämmen, überkämmen, zurückkämmen, hochkämmen, bürsten, in Ordnung bringen
- sich das H., die Haare kämmen, überkämmen, zurückkämmen, hochkämmen, bürsten, in Ordnung bringen
- (jmdm.) das H., die Haare machen umgangssprachlich
- sich das H., die Haare machen umgangssprachlich
- sie kämmt ihr goldenes Haar — Heine Ich weiß nicht
- das H., die Haare waschen, spülen, einrollen, fönen, trocknen
- jmdm. das H., die Haare (ab)schneiden, frisieren, legen, toupieren
- sich das H., die Haare tönen, färben, blondieren, bleichen, lacken lassen
- ich lasse (mir) das H., die Haare wachsen
- mein H. wächst (nach), bricht (ab)
- meine Haare wachsen schnell, langsam, fetten leicht, fallen mir ins Gesicht, fliegen im Wind
- meine Haare fallen aus, gehen aus, werden dünn
- jmdm. (freundlich lächelnd) mit der Hand über das H. streichen
- sich aus Nervosität durch das H. fahren
- die Kinder trugen Kränze im H.
- der frißt mir noch die Haare vom Kopf (= ißt so viel, daß ich arm werde)
Note that in the majority of examples given (including references from well-knownGerman authors) Haar is used in singular. Exceptions may be when it is not about the hair on the head as a whole but when something is done with every single hair of them.
Therefore the grammatically correct usage of "Haar" in the meaning of the question would be using it in singular. When teaching German grammar we need to take care that we don't teach colloquial usage even if this may be used and understood widely.
The only grammatically correct usage in the example sentence therefore is:
Sie hat dunkles Haar.
I'll try to contribute another view, since I don't think the previous answers have addressed all the subtleties.
It's already been mentioned that there are cases where only one form is appropriate. Adjectives like "volles" (full), "schütteres" (sparse), "dichtes" (dense), "lichtes" (sparse) that apply to the body of hair as a whole only go with "Haar" in the singular; you'll find some hits for the plural forms on Google, but I'd say that borders on being wrong. On the other hand, anything that treats the hairs individually requires the plural; I think a realiable rule for this is that one would always use the plural in German if one would use it in English: you find hairs in the drain, not hair, and you split hairs, not hair; the same in German.
The cases that allow either usage are the ones where the adjective could be applied either to the individual hairs or to the body of hair as a whole. Examples are "rote(s)", "wellige(s)", "lange(s)", "dunkle(s)".
In a comment, tohuwawohu wrote that "Du hast schöne Haare" sounds quite odd. I disagree; I would probably rather say "Du hast schöne Haare" than "Du hast schönes Haar", and Google also has a lot more hits for the former. If I'm right on that, then that could be seen as an exception from the general rule I described above, since being beautiful seems more like something one would apply to the body of hair as a whole than to an individual hair.
An aspect that hasn't been mentioned yet is that in the cases where both forms are possible, the singular is slightly obsolescent and restricted to more poetic language. For instance, "Ein Jüngling mit roten Haaren" sounds more incongruous than "Ein Jüngling mit rotem Haar" ("Jüngling" being an archaic word), whereas it's hard to imagine a kid coming home from school and telling her parents that there's a new kid "mit rotem Haar" in school.
Perhaps it is worth mentioning explicitly that in this instance German is more forgiving than English:
In English, the use of the plural usually implies that the hairs (!) in question are from a different body part than the head. In German it doesn't.
NB: Including animal hair in this discussion would open an even larger can of worms: Kamelhaar is usually singular, Hunde- or Katzenhaare is singular only in compounds like Katzenhaarallergie. My point: looking for rules about how individual words behave sometimes is futile.
Just remember that "Haar" sounds better in writing and as a non-native it's usually a good idea to use standard if in doubt.
IMHO the only rule applying here is: if you want to refer to certain, "particular" hairs, plural is necessary:
Bei ihm zeigen sich die ersten grauen Haare.
In every other case i can think of, the singular form is fine, and it's even better in matters of style:
Sie hat dunkles Haar.
sounds better to me than
Sie hat dunkle Haare.
Wictionary tells that "Haar" is a singulare tantum - i'm not sure about it, because there's also a plural form commonly used besides the singular.
It's really hairsplitting if you use
Er mag Frauen mit rotem Haar/roten Haaren (Google-Hits are nearly 1:1)
and imho more a question of convention/linguistic evolution which one is more used. In English hair seems more common ("Hair" musical comes to my mind)
Haare machen
also wins vs. "Haar machen" in Google
volles Haar (thick & dense hair)
is a very common phrase (volle Haare not!)
jemandem kein Haar krümmen
a common idiom. Here plural makes no sense as you want to point out you didn't bend a single hair. Same with Haarspalterei
A interesting similar case are the
Wimpern (eyelashes)
Here the plural is normally used in most formulations. So I don't see any grammar rule solving this "issue", when Haar/Wimpern has to be used in Singular/Plural (as objects with same structure), again it's hairsplitting to me...
If the adjective can only refer to a collection of hairs, then singular is usually obligatory.
Example:
schütteres Haar
sparse hair
If the adjective can both refer to a single hair or to the collection of hairs, then both forms are possible and may convey additional information, in particular, the singular has a more detached flavor.
In informal speech, the singular is practically only used in idioms.
The Udo Jürgens song "17 Jahr, blondes Haar" is a very poor counterexample, because noone uses "Jahr" instead of "Jahre", either, when the meter of the poem does not demand it. Compare the song lyrics "Wir haben noch Wind in den Haaren, Wind von Bergen und See.", for a situation where rhyme and meter does not demand the singular.