This is one for the people who are interested in languages.
So, German, like some other languages, assigns "gender" to nouns. You can tell the gender from the article, which therefore should memorize the article with the word, if you ever have to memorize German vocabulary. "Der" is male, "die" is female, "das" is neuter.
Tables and the sky are male, vases and traffic lights female, books and windows neuter. There may be some weird logic behind it when you go far enough back in time, or maybe not, but it looks pretty arbitrary, and it has for most words nothing to do with biological sex. Since the same goes for animals, which often do have a sex, so that gets a bit confusing.
the dog = der Hund
the cat = die Katze
the horse = das Pferd
In consequence, any dog (and frog, and most species of bird) of unknown sex is referred to with "er" ("he"); any cat (and spider) of unknown sex is referred to as "sie" ("she"), and any horse of unknown gender is referred to as "es" ("it").
You cannot just change the article to change the gender. "Die Pferd" isn't a female horse, it's a mistake. The word for a mare is "Stute". And, yes, that is grammatically female, too; at least that much logic is in the system. Incidentally, a gelding is "der Wallach" - castrating a stud still leaves him male. Some people get weird ideas in that department.
One rather prominent example where the biological sex doesn't match the grammatical gender is the German word for "girl", that is, "das Mädchen". That's because that word is a diminutive (of "Magd", meaning "maid"), and all diminutives are neuter.
There are some words that can be more than one gender. "Gelee" ("jelly") can be male or neuter, depending on whom you ask. Other words mean different things depending on gender. I don't know many examples, but here they are:
"Die See" is the sea, while "der See" is a lake.
"Das Tau" is a rope, "der Tau" is dew. (As far as I can tell that's a coincidence, two different etymologies arriving at the same syllable.)
If someone says she's driving to Shanghai "mit ihrer Honda" ("with her(female) Honda"), she's going by motorbike, if she says she's driving "mit ihrem Honda" ("with her(male) Honda"), it's a car.
So, "the Harley" is female. "The motorbike" is neuter. That's because the German word, "Motorrad", is a compound noun formed from "der Motor", which means what you think it means, and "das Rad", which means "wheel".
This is one of the few hard-and-fast rules: In compound nouns, the gender is always that of the last component noun.
I've been trying to find some other patterns, but
Nouns formed with -heit or -keit at the end are always female - "die Dummheit" (stupidity, stupid idea), "die Wirksamkeit" (effectiveness), "die Menschheit" (humanity)
Talking about trees: There may be certain rules of the thumb you can find. While "der Baum" (the tree) is male, pretty much any species of tree whose name does not end in "-baum" and which is native to Europe is female - die Birke (birch), die Eiche (oak), die Buche (beech), die Fichte (spruce)... "Exotic" trees on the other hand are more commonly male: der Ginko, der Eukalyptus, der Baobab.
A similar pattern can be found with river names, although a bit more muddied. There are some male rivers around here, most prominently the Rhine, but a lot of rivers in Europe are female (including the Loire, Thames, and Volga). On the other hand I can't came up with a female river outside of Europe. The word for river, "Fluss", is male, too, so that might have something to do with it.
I'll leave it at that for now, hoping conlangers or writers or just apprentice language geeks get something out of it.
