1
I was playing with a scissors and I cut my finger.
I was playing with scissors and I cut my finger.
I was playing with a pair of scissors and I cut my finger.
Scissors is a plural noun. Do not say a scissors. Use scissors or a pair of scissors.
2
This scissors is very sharp.
These scissors are very sharp.
This pair of scissors is very sharp.
Because scissors is plural, use these/those and a plural verb: ‘These scissors are very sharp.’
The noun pair is singular, so use this/that and a singular verb: ‘This pair of scissors is very sharp.’
Other nouns normally used in the plural include trousers, shorts, jeans, pyjamas and glasses.
See PYJAMAS