Languages have nouns that are either masculine or feminine. German adds a third gender: neuter. The masculine definite article (“the”) is der, the feminine is die, and the neuter form is das. Germans have had many years to learn whether wagen (car) is der or die or das. It's der wagen.
Words in these categories is always "der" (the) or "ein" (a or an).
Days, months, and seasons: Montag, Juli, sommer (Monday, July, summer). The one exception is das Frühjahr, another word for der Frühling, spring.
Points of the compass, map locations and winds: nordwest(en) (northwest), süd(en) (south), der föhn (warm wind out of the Alps), der scirocco (sirocco, a hot desert wind).
Words in these categories are das (the) and ein (a or an):
Nouns ending in -chen or -lein: fräulein, häuschen, kaninchen, mädchen (unmarried woman, cottage, rabbit, girl)
Names of hotels, cafés or theaters.
Names of colors used as nouns: das blau, das rot (blue, red).
Feminine words take the word "die" (the) or "eine" (a or an).
Nouns ending in -heit, -keit, -tät, -ung, -schaft: die gesundheit, freiheit, schnelligkeit, universität, zeitung, freundschaft (health, freedom, quickness, university, newspaper, friendship). .
Names of aircraft, ships, and motorbikes: die Boeing 747, die Titanic, die BMW (motorbike only; the car is der BMW). The die comes from die maschine, which can mean plane, motorbike, and engine.