Every German noun belongs to one of three grammatical genders: masculine, feminine, or neuter. The definite article reflects this gender: "der" for masculine nouns, "die" for feminine nouns, and "das" for neuter nouns. For example, "Tisch" (table) is masculine, taking the article "der Tisch"; "Lampe" (lamp) is feminine, taking "die Lampe"; and "Buch" (book) is neuter, taking "das Buch." Other everyday nouns include "der Computer" (computer) and many food words covered later. Unfortunately, there is no single rule for assigning gender, so the article must usually be memorized along with each noun.
That said, certain suffixes provide reliable clues about gender. Any noun ending in the diminutive suffix "-chen" is automatically neuter, so "Mädchen" (girl) is "das Mädchen." Nouns ending in "-heit" are always feminine, as in "die Freiheit" (freedom). Similarly, nouns ending in "-tät" are feminine, so "die Universität" (university) takes the feminine article. Spotting these suffixes is a quick way to predict a noun's gender before memorizing it.
The indefinite article also changes with gender. For masculine and neuter nouns, the form is "ein," as in "ein Mann" (a man) or "ein Kind" (a child). For feminine nouns, the form is "eine," as in "eine Frau" (a woman). Interestingly, in the plural, German uses a single article "die" regardless of whether the singular noun is masculine, feminine, or neuter, so the uniformity of the plural articles simplifies one aspect of the language considerably.