1
I’ve bought a new pyjama.
I’ve bought a new pair of pyjamas.
Pyjamas, or pajamas in American English, is a plural noun with no singular form. Use a pair of pyjamas when talking about one set: ‘I need a new pair of pyjamas.’
2
I don’t like this pyjamas.
I don’t like these pyjamas.
Because pyjamas is plural, use these/those, a plural verb and a plural pronoun: ‘These pyjamas are comfortable.’ ‘Where are my pyjamas? I can’t find them.’
The singular form pyjama or pajama can be used before another noun: ‘a pyjama top’, ‘pyjama bottoms’.
See SCISSORS