Just to make things more complicated, certain masculine nouns are “weak” and take an “n” ending in all cases except the nominative. For example, most of the words for “boy” in German (Junge, Bursche, Knabe, Bube) fall into this group:

There are at least a few hundred weak nouns and it’s impractical to memorize them all, but once you know some of the most common ones, you can start to recognize them. They fall into two basic groups. The first, which you just saw, end in e and usually refer to people or animals:

These aren’t hard to remember, because there are very few masculine nouns in German that end in a single e and are not weak. Der Käse (cheese) is the only common one we can think of.

The only wrinkle with this group is that a few of them keep the genitive s after the n. These are often the ones that don’t refer to a person or animal, like der Wille (will, volition) or der Gedanke (thought, idea). So the genitive of those would be des Willens and des Gedankens respectively. And the only non-masculine weak noun also works this way: das Herz (heart) --> des Herzens.

The second group is a little fuzzier, but it’s basically nouns with certain Latin and Greek endings. Most of them are so close to their English equivalents that we don’t even have to translate:

Finally, there are a few weak nouns that don’t fit into either group. The most common are:

You can find long lists of weak nouns, but you shouldn't try to memorize them all. If you remember the basic types above, you’ll get most of them right, and if you don’t, it’s not a real barrier to comprehension anyway.