In Arabic, it is important to know the gender of a noun because this affects the construction of verbs, adjectives and pronouns in the rest of the sentence. There are masculine  and feminine nouns. Certain signs indicate the gender of a noun automatically, but sometimes there are no signs and this is where memory must come into play.

1. If a word contains then it is usually feminine:

If a word denotes a biological feminine, then it is also grammatically feminine. Common sense, I suppose!:

2. In other cases one just has to memorize which words are masculine and which are feminine. The Quran e Majeed remains the most authentic source of gender usage.
An interesting point to note is that all parts of the body that come in pairs are always feminine, eg. eyes and ears.

Masculine Nouns

 Feminine Nouns

Mutanabbi, the legendary Arabic poet, plays on this grammatical peculiarity in one of his poetic works:

If all women were like the one we have lost, then women would be superior to men. It is not demeaning for the sun to be a feminine noun; just as it is not a matter of pride for the moon to be a masculine noun.

Syedna Idris Imaduddin (RA) quotes this verse in praise of Maulatuna Hurratul Malika (AS), who though a woman, surpassed many Hudud Fodala in her ilm, khidmet and ikhlaas.