de is "they", unless it is used as a definite article immediately before an adjective for a plural noun, in which case it is "the". E.g. de gröna bilarna = the green cars. det is either third person singular neuter "it" or the definite article immediately before the adjective for a singular neuter (et) noun. Furthermore, den is either third person common gender (en) or the definite article for singular common nouns with an adjective.