But there are rules for 2/3 of cases. https://sprachekulturkommunikation.com/genus-der-substantive...
You can classify by suffix.
* -ung, -heit, -keit -> feminin, e.g. die Schönheit
* -ling -> masculin, e.g. der Flüchtling
* -chen, -lein -> neutrum, e.g das Mädchen
You can classify by category. Every alcoholic drink is masculin, except for beer.
You can classify by phonetic spelling. That is probably the closest you have to Portugese.
Just imagine if someone studied Portuguese but learned vocabulary like this, never bothering with the ending vowel:
'gat-'
'cas-'
'bolach-'
Similarly, 'die' should be considered an inherent part of 'Frau'. So don't learn just 'Frau', learn 'die Frau'. The article 'Die' is just as "random" as '-o' or '-a' is in Portuguese. (I'll skip the part where you can have a form of the word in both classes: gata/gato.) People like to try and find "rules" they can remember instead, but it's a pointless endeavor. Language is a Calvinball game.To make a weird tech analogy: Romance nouns are like laptops, with a touchpad built in. Germanic nouns are like desktops, you have to remember to carry a mouse* along.
* Die Maus
It is ok to guess the sex of an animal wrong. The same way nobody (should) cares whether an infant is a boy or a girl.
You can enforce to be neutral be refering to "das Tier" and "das Kind". But since it is just a grammatical sex, why should anyone bother?
There are some culturally assumed defaults:
* a nurse is assumed to be feminine, unless the distinction is important
* a cat is assumed to be feminine, unless the distinction is important