Only if you give no singular article/descriptor or adjective such as "
a pair of blue knickers is lying on the floor", would the singular form of the verb be used.
Most other cases would be more likely to be plural.
"some" or "my" or any other descriptor, even the definite article "the" are all ambiguous so could, and maybe even should, be followed by "are", or "were". The clue is that "knickers ends in an "-s" which makes them plural even though they are only one garment.
So "Your green knickers were/are still on the line."
But knickers or pants or panties are a real oddity in English. In other languages they are one singular word,
Ditto "shorts", "trousers", "jodphurs" etc.
Even where the French pinch our word, they make it singular, hence, "un slip" (which of course they got wrong, as a "slip" in English although still an undergarment is something entirely different,) means wimmins knickers or panties, same as "une culotte".
They use "un short" for a pair of shorts.
(Can't be assed to find out if it's the same in other languages.)
and here endeth the lesson.
WAKE UP AT THE BACK!!!!!!