"He likes the girl."
Translation:Er mag das Mädchen.
Normally you would expect a word meaning 'girl' to be feminine, like 'Mann' is masculine, 'Frau' is feminine etc. But Mädchen is a diminutive (i.e. with an ending that means little) - in German diminutives (usually -chen or -lein) are always neuter. So 'die Frau' (woman) but 'das Fräulein' (young woman) and 'das Mädchen'