What is the difference between the following two sentences?
He has been absent for the past few months.
He has been absent for past few months.
Only the first sentence is grammatical.
Using past and few together sets a very particular and definite time span, so the definite article is obligatory.
There are cases where it's not with other prepositions and adjectives:
in recent months
for a few months
past months had been different
Etc.
However, the combination of for, past and few in your example makes the definite article necessary.