If the unaffected parent is heterozygous (a carrier) for the diseased trait, then some of the offspring will get two copies of the diseased gene and will, therefore, be affected.
(the unaffected parent; is; heterozygous (a carrier) for the diseased trait) (some of the offspring; will be affected; )
the unaffected parent some of the offspring
is will be affected
heterozygous (a carrier) for the diseased trait