Questions & Answers about Obrzęk i sztywność są mniejsze.
Because the subject has two things joined by i (and): obrzęk + sztywność.
So Polish uses the plural verb:
- obrzęk jest mniejszy = the swelling is smaller / less severe
- sztywność jest mniejsza = the stiffness is smaller / less severe
- obrzęk i sztywność są mniejsze = the swelling and stiffness are smaller / less severe
Just like in English, two coordinated subjects normally take a plural verb.
Because mniejsze is the plural form used for a non-masculine-personal subject.
In Polish plural agreement works differently depending on whether the group is masculine personal (adult men, or a mixed group including men in many contexts) or non-masculine-personal (things, animals, abstract nouns, women-only groups, etc.).
Here the subject is:
- obrzęk — masculine inanimate singular
- sztywność — feminine singular
Together they form a non-masculine-personal plural subject, so the predicate adjective is:
- mniejsze
Compare:
- te objawy są mniejsze = correct type of plural agreement for things/symptoms
- mniejsi would be used for masculine-personal plural, for example with people