The gender is indicated by the definitive article: der - masculine, die - feminine, das - neuter. Why is tree masculine, the mouse feminine and the heart neuter? We don't know. There are no rules for that. Therefore it is always recommended to learn the nouns together with their article. But: In a few cases however, there are rules, e.g.: Der Sommer (the summer) - Names for seasons, months and days are always masculine. Die Sonne (the sun) - Many nouns ending in -e are feminine. Das Schweinchen (the piglet) - All nouns ending in -chen or -lein are neuter. A few rules: macsuline: male persons and professions (der Mann, der Koch) time: days, months, seasons many nouns with the ending -er (der Computer, but: die Nummer), all nouns with the ending -ling (der Liebling), all nouns with the ending -or (der Motor), all nouns with the ending -ismus (der Kapitalismus) feminine: female persons and professions (die Frau, die Lehrerin) many nouns with the ending -e for example die Sonne, die Reise (but: der Käse); all nouns with the ending -ung (die Rechnung), -heit (die Sicherheit), -keit (die Sauberkeit), -schaft (die Freundschaft), -ei (die Zauberei), -ion (die Situation), -ur (die Natur), -ät (die Realität); neuter: all nouns with the ending -chen (das Mädchen), -lein (das Männlein), -um (das Museum), -ment (das Instrument) nouns from the infinitve of the verb (das Essen)

There are some rules, but as well as many exceptions. There are some sites in German with rules about the gendr of nouns and in brackets you get the probability in %. There you can see 50, 60, 70%, sometimes 90%. That means that it is only of limited value to learn the rules. Learn some basic rules, otherwise always learn the noun together with the article. I can send you some links in English: https://www.fluentin3months.com/german-noun-genders/ https://en.wikipedia.org/wiki/Grammatical_gender_in_German https://www.deutsch-als-fremdsprache-lernen.de/deutsche-grammatik-substantive-grammatisches-geschlecht/

Manchmal gibt es dafür Zeichnen. Beispielweise alle Wörter, die mit der Selbe "chen" enden, bekommen das Artikel das. Man bekommt eher ein Gefühl dafür mit der Zeit, weil man sehr schwer auswendig lernen kann. for example if a word has the end chen, then it goes with das. there are long lists with the different possibilities and ends. you are going to develop a feeling for der, die and das with time. i am not sure if that is that was your question