"In winter, you wear warmer pajamas."
Translation:En hiver, tu portes un pyjama plus chaud.
I do not understand why this must be un rather than le or even du. 'A' would be my last choice after "the warmer" or "some warmer." Overall, the sentence seems to me more like a sort of general rule about the people in this climate, that they wear warmer pyjamas in the winter. So I went with on porte, which was OK, but got 1 where I put le.
You are probably confused because "pajamas" is always used in plural and therefore as a bare noun in English. If this noun were regular, with a singular and a plural form, this case would be clearer.
- You wear a warm scarf = tu portes une écharpe chaude
- You wear warm scarves = tu portes des écharpes chaudes
In French, just like "un pantalon, un jean, un short", etc. "un pyjama" is a regular, singular or plural noun.
- You wear warm pajamas = Tu portes un pyjama chaud OR tu portes des pyjamas chauds