For anyone else wondering about that 'De':

When an adjective or numeral is used in front of a noun with the definite article, an additional definite article is placed before the adjective(s). This additional definite article is det for neuter nouns, den for common nouns, and de for plural nouns, e.g. den nya flaskan (the new bottle), det nya brevet (the new letter), de fem flaskorna (the five bottles).

Source: http://en.wikipedia.org/wiki/Swedish_grammar#Articles_and_definite_forms

whats with the "de" and the "the" added to byxor

Can one use honom here?

Why can't I use sin or sitt here?

Is it correct to replace sin instead of hans??