German adjectives really aren't that hard; they just look that way
I may have scared a bunch of people with that chart of German adjective endings, but as several commenters noted, native speakers don’t refer to the charts; they just say what comes naturally. (Well, except for Leo Petr, who claims that native Russian speakers actually study these charts in grade school.) Commenter Helga Waage noted that one quickly sees patterns in the charts that make them much easier to digest. And that’s true. But I taught myself the German adjective endings a completely different way. If you’re a student of German, you might find this helpful. If you’re not, then you probably just want to skip the rest of this entry.
As a side note, you have to make sure you put the columns in the right order. In many textbooks, the columns are ordered as “masculine, feminine, neuter, plural”, but this fails to highlight the strong similarity between the masculine and neuter genders. From a grammatical standpoint, German neuter nouns are “90% masculine, 10% feminine”; therefore, it’s more natural to put the neuter column between the masculine and feminine columns. I therefore prefer the order “masculine, neuter, feminine, plural”, which as it so happens appears to be the order that Germans themselves use.
I’m going to do away with the terms “strong”, “weak”, and “mixed”. Instead, I’m going to reduce it to the question “How much work does the adjective have to do?” which breaks down into two inflections. In my mind, I don’t have terms for these two inflections, but for the purpose of this discussion I’ll call them “hardworking” and “lazy”.
We start with the lazy inflection, which is used when the definite article or a word that has the same ending as the definite article is present. The lazy inflection is simple: In the singular of the nominative and accusative cases (the “easy cases”), the ending is “-e”. In the plural and in the genitive and dative cases (the “hard cases”), the ending is “-en”.
There is only one exception to this general rule, which I highlighted in the table above. But even that exception is natural, because the masculine gender is the only one whose articles change between the nominative and the accusative, from “der” to “den” and “ein” to “einen”, so you’re already used to sticking an extra “-en” in the masculine accusative singular.
(By the way, I call the nominative and accusative the “easy” cases since most textbooks teach them them within the first few weeks, which means that you’ve quickly become familiar with them and treat them as old friends. On the other hand, the dative and genitive are not usually introduced until second year, thereby making them “hard” due to their relative unfamiliarity.)
The hardworking inflection is even easier than the lazy inflection. You use the hardworking inflection when there is no word that has the same ending as the definite article. In this case, the adjective must step up and take the ending itself. (I’ve included the definite article in the chart for reference.)
Hey, wait, I left two boxes blank. What’s going on here?
Well, because in those two cases, even if there is nothing else to carry the ending of the definite article, the noun itself gets modified by adding “-s”. For example, the genitive of the neuter noun “Wasser” (water) is “Wassers” (of water). The word that carries the ending of the definite article is the noun itself! That’s why I leave the boxes blank: The scenario never occurs in German.
It is those empty boxes, however, that always trip me up. When it comes time to decide what ending to put on the adjective, and I’m in one of those two boxes, the word with the ending of the definite article hasn’t appeared yet so I think I’m in the “hardworking” case. And then when I get around to saying the “-s” at the end of “Wassers”, I realize, “Oh, crap, there’s that indicator. I should have used the lazy form.” But it’s too late, I already said the adjective with the wrong ending. I could go back and fix it, but that would interrupt the flow of the conversation, so I usually decide to let it slide and take the hit of sounding stupid. (Or, more precisely, sounding more stupid.) If you listen carefully, you may notice me pause for a fraction of a second just as I reach the “-s” and the realization dawns on me that I messed up again.
If you compare my charts to the official charts with strong, weak and mixed inflections, you’ll see that my “lazy” inflection matches the weak inflection exactly, and my “hardworking” inflection matches the “strong” inflection except for those empty boxes. (Because, under my rules, those empty boxes are lazy.) The mixed inflection matches the “lazy” inflection except in three places, which I count as “hardworking” because the indefinite article “ein” does not take an ending in exactly those three places.
Anyway, so there’s how I remember my German adjective endings. Mind you, I don’t work through the details of these rules each time I have to decide on an ending. I just have to make the simple note of whether the definite article ending has already appeared (or in the case I always forget: will soon appear). If not, then I put it on the adjective.