usually not. You need to guess either cat or tomcat, that designates a grammatical sex. All references are based on grammatical sex. It is strongly emphasized that a grammatical sex is not the same thing as a natural sex.

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

From a foreign learners perspective, it is easier to just learn the article together with the noun.

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.

The gender of a noun is just a noun class. But because Germanic languages lie more toward the analytic end of the morphological typology continuum (whereas Romance languages lie more toward the synthetical end) the information is latent - or rather, the task of conveying that information is left to other words (the articles).

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

I guess that beginner text books need to focus on teaching you one feature at a time.

It could be better if you aim for C1 level text books. But since free exercises are hard to check, it is probably not meant for a book.