Noun, feminine
You now know that Wassergefährdungsklasse is feminine. But what about all the other German words? Do you have to learn the article of every single word by heart?
Unfortunately, German has only a few general rules that help you work out a noun's article. Grammatical gender in German is often not intuitive at all, and a thing's natural gender rarely helps. Girls, for example, are naturally female, yet the word “Mädchen” is neuter: you say “das Mädchen”.
So if you want to be sure of a word's gender, you have to learn it by heart.
Luckily, German does have a few gender rules that make things a bit easier. Of course it would be even better if those rules had no exceptions – but you can't have everything!
Colours – such as “das Gelb” or “das Rosa” – are almost always neuter. So are nouns ending in -chen / -tum / -ment: das Märchen, das Stadium, das Testament.
Many words to do with time and dates – der Mittwoch, der August, der Winter – are masculine. Many weather nouns are masculine too: der Wind, der Regen, der Hagel. Most alcoholic drinks (except “das Bier”!) take “der”: der Schnaps, der Champagner, der Gin. Words ending in -ig / -ling / -ich / -en are almost always masculine as well, for example der Bahnsteig, der Wüstling, der Rettich and der Hafen.
Almost all kinds of fruit (apart from a few such as “der Apfel” or “der Pfirsich”) are feminine. Many suffixes also appear almost only on feminine words: -in / -heit / -schaft / -ei / -keit / -ung / -ion, as in die Rektorin, die Anwesenheit, die Belegschaft, die Fischerei, die Einsamkeit, die Zeitung, die Adaption.
If you know a noun's gender, you also know its indefinite article. German has only two: ein and eine. Feminine nouns take “eine”; everything else takes “ein”. So it is eine Wassergefährdungsklasse.
You use an indefinite article when you are not quite sure (or don't need to be) exactly who or what you are talking about.
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