The original meaning of the Tagalog word minsan is 'once.' It is now commonly understood in conversation to mean 'sometimes' as a short version of paminsan-minsan (from time to time).
Minsan minahal kita.
I once loved you.
from time to time, occasionally, sometimes, now and then
Sometimes I have doubts.
Mahalin mo ako minsan pa.
Love me once more.
used to describe a one-time thing
This opportunity comes once.
Minsanan lang ang ganitong pagkakataon.
This kind of opportunity comes only once.
miminsan (not common)
Ang kasalungat ng salitang minsan ay parati o lagi.
The opposite is the word 'sometimes' is 'all the time' or 'always.'