Learning the gender of German nouns can be a problem for English-speakers. After all, we aren't used to nouns having any gender at all. ("She's a good ship" is a rare exception.) But as I explained previously in German Gender Hints, there are ways to predict the gender of a German noun. Not all nouns are truly predictable, but many are.
But even when there is a rule to help determine the gender of a given noun category, there are almost always exceptions. For instance, certain noun endings or suffixes in German can tell you a noun's gender. Some, however, are more reliable than others. Take the example of German nouns ending in -o. Such wordsoften English cognates borrowed from Latinare usually neuter: das Auto, Büro, Kasino, Konto, Radio, Veto, Video, usw. But in this o-suffix category about one in three of the nouns is an exception to the rule. Some of the exceptions are very common: die Avocado, der Euro, die Limo, der Zoo. A sharp observer will notice that most of the exceptions are either not from Latin (die Avocado) or are short forms of longer words that determine the gender (die Limo, short for die Limonade).
German nouns ending in -ik are usually feminine: die Grammatik, Grafik, Klinik, Mathematik, Musik, Physik, Panik. This is a fairly reliable predictor, but again there are some common exceptions, including der Atlantik, der Pazifik, der Katholik, and das Pik (spade, cards). Most of the few -ik exceptions are logical, particularly the two oceans, since the German word for ocean is der Ozean.
Another German feminine noun suffix that is very reliable for predicting gender is the -in ending. This suffix is the most common way to turn a German masculine "people" word or occupation into a feminine noun (der Architekt/die Architektin, der Lehrer/die Lehrerin). However, there are a few nouns ending in -in that are not feminine: das Aspirin, der Harlekin, das Benzin, der Urin. But you'll notice that the -in exceptions are usually non-people words.
An -er ending usually indicates a masculine noun. Such nouns are usually agents (people who do things), nationalities, and professions. However, some common nouns ending in -er are not masculine: das Fenster, die Mutter, die Schwester, die Tochter, das Wetter.
There are many other examples of gender predictors in German Gender Hints, but you can find a glossary with the full English and German for the noun categories mentioned here on the following pages. Just remember this: the best way to know the gender of a German noun is to learn it along with the word!