My dictionaries list both genug and genügend as sufficient, enough etc.
Are there any differences between the two other than one functions as a participle of genügen?
Additionally, are these two interchangeable?
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Broadly speaking, both words can be used as adjective but only genug can be used as adverb. Sharing their meaning and being interchangeable, they mean ausreichend, or hinreichend and can be translated with sufficient and enough.
I'd say that you can replace genügend with genug (almost) always but not vice versa. I think there's a slight tendency to use genügend when referring to an uncountable noun while people rather use genug when it's countable.
Er hat nicht genügend[=genug, ausreichend] Acht gegeben.
Ich habe noch genug[~viel] Arbeit vor mir. (genügend quite rare)
You cannot use genügend as adverb.
Ich habe genug gesehen. (not genügend)
Er ist alt genug. (not genügend)
You can place genug before and after a noun but genügend must precede the noun.
Er hatte Zeit genug um sich auf die Prüfung vorzubereiten.
Er hatte genug Zeit um sich auf die Prüfung vorzubereiten.
Er hatte genügend Zeit um sich auf die Prüfung vorzubereiten.
Er hatte ausreichend Zeit um sich auf die Prüfung vorzubereiten.
When you search the Internet you will find some notes that genug is a particle (namely Gradpartikel) and it's also used as indefinite pronoun (like some, many etc.).
Semantically speaking, that's correct but this is a finer point of language analyzing and does not help you to fully understand its usage at all.
The words have slight differences in meaning and usage, but are partly interchangeable to a given extent:
First of all "genügend" is used less often, because it is more part of literary language, and "genug" also part of colloquial language.
Independant from that, "genügend" is best translated as "sufficient" in the sense of "adequate", and often has a reference (enough wood to make fire, enough money to avoid starving). For me this is quite independant from countable vs. uncountable: "genügend Wasser (zum Schwimmen)", "genügend Spieler für zwei Mannschaften"
"genug" means "enough". It has kind of a second, more absolute meaning: not only sufficient, but really enough: ("Do you have enough?", "Now, it's enough!")
So, sometimes "genug" is stronger and can easier replace "genügend" than in the other direction.
Both can be used as an indefinite pronoun (as "some") (only- as mentioned before, "genügend" is used rarer, but generally):
Genügend Menschen sind für Naturschutz, dass dies ein Thema aller Parteien wurde.
Es gibt nicht genug Menschen, die sich im Naturschutz engagieren.
(In both cases, the words are interchangeable, but there is a semantic preference to use them as written.)
To make a point, I would say, and I think I speak about a majority of people here:
I verdiene genügend Geld, aber nicht genug.
genügend is also used as an Adverb. In fact, in german language, when
genügend is in front of a noun, they consider it an adverb.