Are there rules of when there is linking-sound compound words?

Question

I've started refreshing my german knowledge and using Duolingo. However I seem to constantly struggle with the compound nouns or rather the linking between the parts. Are there rules for which to use if any?

Examples:

• der Blumenladen: Blume+n+Laden, there's a linking "n" there, but not on der Buchladen.
• die Geburtstagskarte: Geburt+s+Tag+s+Karte, there's a linking "s" there
• die Muttersprache: Mutter+Sprache, there's no linking there

Or even there's sometimes apparent chopping of parts:

• die Erdbeere: Erde-e+Beere (i.e., the last "e" in Erde dropped out).

Answer 1

Those Fugenlaute are all about easing pronounciation. So the only rule about that is:

• Is it hard to pronounce? — Don't write it that way then.

If there are multiple ways that are easy to pronounce you can be sure all of them are used but at some point almost all speakers settle on one or two variants. That's the ones that go to the dictionaries. Often that depends on the region as well. E.g.

• Schwein + Braten → Schweinebraten (Northern)
• Schwein + Braten → Schweinsbraten (Southern)

Answer 2

Sometimes these are called Fugenlaute or Furgenmorpheme, but I think a better way of thinking about them is in terms of Kompositionsstammform, i.e., a base form of a word (noun) that is used when the word appears in the forst position of a compound words. (see, e.g., https://www.researchgate.net/profile/Sascha_Michel/publication/282074292_Zur_Systematik_der_Kompositionsstammformbildung_bei_NN-Komposita_-_Implikationen_fur_den_DaF-Unterricht/links/56025e0c08aeb30ba7355f1b/Zur-Systematik-der-Kompositionsstammformbildung-bei-N-N-Komposita-Implikationen-fuer-den-DaF-Unterricht.pdf)

Note that for most words, compounds are always formed with the same form, regardless of the next sound:

• Sonnenmilch, Sonnenaufgang, Sonnenschein, Sonnentag
• Blumenladen, Blumenwiese, Blumenstrauß, Blumenerde
• Liebesbrief, Liebesaus, Liebesbeweis, Liebestod
• Kompositionsstammform, Kompositionsforschung, Kompositionsergebnis
• Hundehütte, Hundeschnauze, Hundeangriff
• Zahnbürste, Zahnrad, Zahnausfall, Zahnschmerzen
• Institutsarzt, Institutsdirektor, Institutskrankenhaus
• Hautarzt, Hautdrüse, Hautkrebs

In the last two lines, the meeting sounds are the same (t-a, t-d, t-k), but one of them has an -s- and one doesn't -- that's because compounds with Institut are alway Instituts-x, while Haut+x compounds go without -s-. So it's not generally about which sound combinations are easier to pronounce.

For some of those, the base form looks like plural of genitive, but that is coincidence. For example, compounds that start with Sonne are always Sonnen-x, even though they usually refer to one sun. Compounds with Zahn always are Zahn-x, wether they involve a single tooth (Zahnschmerzen) or many (Zahnbürste (for most of us at least)). Some of the base forms do not correspond to a standard form of the original noun (Liebes, Instituts, Kompositions are not forms of the respective nouns).

Of course, it's more complicated. Some words have several compound base forms. Some word have different forms in local language varieties, or in certain professional contexts such as law.

But in a first step, you could try to learn the (usually) correct base forms for each noun.

Answer 3

Q: Are there rules of when there is linking-sound compound words?

A: Yes, there are rules. But they are numerous, weak and full of exceptions (which, of course, are themselves also numerous weak rules full of exceptions).

Those linking-sounds are called Fugenlaute (singular: Fugenlaut = joint-sound). They sometimes derive from a plural form, sometimes from a genitive form and sometimes from some other form of a word, but sometimes from no existing form at all. For example, there is an s in Liebeslied, but the noun Liebe has no grammatical form with an s at the end.

Here is a derivation from a plural form:

• Gästehaus = a guest house, i.e. a house that contains rooms or apartments for guests, with no other use than hosting guests

But sometimes there is also a singular version of the same word:

• Gasthaus = an inn (restaurant) where you can drink and eat

Sometimes there is a form derived from plural, and also a form derived from singular, sometimes even with different linking-sounds which all mean the same:

• Mäuseloch = Mauseloch = Mausloch = mouse hole

Sometimes the linking sound depends not only on the first word, but also on the second:

• Männerfreundschaft (but not Mannesfreundschaft)
• Manneskraft (but not Männerkraft)

The linking-sound, that has to be used between two given nouns even can vary, depending on the geographical region:

Rhineland	Austria	Switzerland
Speise-en-karte	Speise-e-karte	Speise-e-karte
Schwein-e-braten	Schwein-s-braten	Schwein-e-braten
Jahr-es-zahl	Jahr-es-zahl	Jahr-zahl

Sometimes different linking-sounds change the meaning:

• Kind-er-kopf = head of a child
• Kind-s-kopf = an adult person that behaves childish