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

They are in the nominative case, because they are the subject of the sentence.

You can identify the basic structure as:

• Obrzęk i sztywność — subject
• są — verb
• mniejsze — predicate adjective/complement

So this is the normal subject + to be + adjective pattern.

Yes. Mniejsze is the plural form of the comparative mniejszy / mniejsza / mniejsze, meaning smaller or, in medical contexts, often less severe.

The base adjective is:

• mały = small

Its comparative is:

• mniejszy = smaller
• mniejsza = smaller (feminine)
• mniejsze = smaller (neuter singular or non-masculine-personal plural)

In this sentence, mniejsze means something like:

• less severe
• reduced
• smaller than before

Polish often leaves the comparison implicit when the context is obvious.

So są mniejsze can mean:

• are smaller
• are less severe
• have decreased

Usually the unstated idea is:

• compared with before
• compared with an earlier stage
• compared with another condition/treatment

If you wanted to state the comparison explicitly, Polish can do that too, for example:

• Obrzęk i sztywność są mniejsze niż wcześniej. = The swelling and stiffness are less severe than earlier.

In a medical sentence like this, it usually means less severe or reduced, not literally physically smaller in a geometric sense.

So depending on context, a natural English translation could be:

• The swelling and stiffness are less severe.
• The swelling and stiffness are reduced.
• The swelling and stiffness are smaller.
  (possible, but less natural in medical English)

Polish often uses mniejszy more broadly than English smaller.

In a normal full sentence like this, są is needed.

Polish does not usually omit present-tense być (to be) in standard sentences of this type.

So:

• Obrzęk i sztywność są mniejsze. = normal, correct sentence

Without są, it may sound like:

• a note
• a heading
• a telegraphic style
• something incomplete in normal speech

So for learners, the safe rule is: keep jest / są in this kind of sentence.

Because Polish has no articles like English a/an/the.

So:

• obrzęk can mean swelling or the swelling
• sztywność can mean stiffness or the stiffness

The exact meaning depends on context.

That is why Obrzęk i sztywność są mniejsze can naturally translate as:

• The swelling and stiffness are less severe even though Polish has no separate word for the.

No, Polish word order is fairly flexible, though this version is the most neutral and straightforward:

• Obrzęk i sztywność są mniejsze.

Other orders are possible, but they usually change the emphasis:

• Mniejsze są obrzęk i sztywność.
  This sounds more marked, as if emphasizing what is smaller.
• Są mniejsze obrzęk i sztywność.
  Usually less natural in ordinary speech.

For learners, the given order is the best default.

• obrzęk is masculine inanimate
• sztywność is feminine

You can often see this from agreement in the singular:

• obrzęk jest mniejszy
• sztywność jest mniejsza

But when they are combined, the plural agreement becomes:

• są mniejsze

because together they are a non-masculine-personal plural subject.

Like this:

• Obrzęk jest mniejszy. = The swelling is less severe / smaller.
• Sztywność jest mniejsza. = The stiffness is less severe / smaller.

Notice how the adjective changes:

• mniejszy with masculine singular obrzęk
• mniejsza with feminine singular sztywność
• mniejsze with the combined plural subject

This is a very useful pattern to practice.

A rough pronunciation guide is:

• Obrzęk ≈ OB-zhengk
• i = ee
• sztywność ≈ SHTIV-noshch
• są ≈ sow̃ (nasal vowel, something like so with nasal resonance)
• mniejsze ≈ MNYESH-sheh

A few helpful notes:

• sz sounds like English sh
• rz often sounds like zh as in measure
• ę is a nasal vowel; before some consonants it may sound closer to en/em
• ń is a soft n
• szcz / ść / ź / ń-type sounds often take practice for English speakers

Not literally. Mniejsze specifically means smaller / less severe / reduced, not better by itself.

However, in context, an English speaker might understand the overall idea as an improvement. So:

• literal sense: The swelling and stiffness are less severe / reduced
• implied medical meaning: Things are better

If you want to stay close to the Polish grammar and wording, less severe or reduced is best.

Sztywność is usually an uncountable abstract noun, like stiffness in English.

So in this sentence it is singular because it refers to the condition itself, not to separate individual units.

Similarly:

• obrzęk can also function as a general medical noun

So the sentence talks about two conditions:

• swelling
• stiffness

not about multiple separate swellings or multiple stiffnesses.