Mięsień i ścięgno bolą.

Because the sentence has a compound subject: mięsień i ścięgno = the muscle and the tendon.

In Polish, when two singular nouns are joined by i (and), the verb usually goes into the plural:

• Mięsień boli. = The muscle hurts.
• Ścięgno boli. = The tendon hurts.
• Mięsień i ścięgno bolą. = The muscle and the tendon hurt.

So bolą is the 3rd person plural form of boleć.

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

• mięsień = nominative singular
• ścięgno = nominative singular

The structure is basically:

• [subject] + [verb]
• Mięsień i ścięgno + bolą

This is different from sentences like Boli mnie ręka (My arm hurts / literally The arm hurts me), where Polish often uses a different structure.

i means and.

So:

• mięsień i ścięgno = muscle and tendon

It simply joins the two nouns into one subject.

Polish does not have articles like English a / an / the.

So:

• mięsień can mean a muscle or the muscle
• ścięgno can mean a tendon or the tendon

The exact meaning depends on context.

In English, you must choose:

• A muscle and a tendon hurt
• The muscle and the tendon hurt

In Polish, the same sentence can cover both, depending on the situation.

The dictionary form is boleć, which means to hurt, to ache, or to be painful.

Here are some useful forms:

• boli = hurts (singular)
• bolą = hurt (plural)

Examples:

• Ząb boli. = A tooth hurts.
• Zęby bolą. = Teeth hurt.
• Mięsień i ścięgno bolą. = The muscle and the tendon hurt.

A rough English-friendly guide:

• Mięsień ≈ MYEN-shyen
• i = ee
• ścięgno ≈ SHCHYENG-no (the first cluster is tricky)
• bolą ≈ BOW-lon (with a nasal final vowel)

A few pronunciation notes:

• ę is a nasal vowel.
• ś sounds like a soft sh.
• ci before a vowel often sounds somewhat like a softened ch/ty combination.
• ą is also nasal.

If you want a simpler approximation for beginners, you could think of the whole sentence as:

MYEN-shyen ee SHCHYENG-no BO-won

Not perfect, but close enough to get started.

• mięsień is masculine
• ścięgno is neuter

This matters because Polish nouns belong to grammatical genders, and that affects agreement in many situations.

In this sentence, though, the main thing to notice is that the subject is mixed and joined by i, so the verb is simply plural: bolą.

Yes. Polish word order is more flexible than English because the endings show grammatical function.

These are all possible, though not always equally natural in every context:

• Mięsień i ścięgno bolą.
• Bolą mięsień i ścięgno.

The first one is the most neutral and straightforward for a learner.

Changing word order can affect emphasis more than basic meaning.

Yes, in standard Polish that would be wrong.

Since the subject has two parts joined by i, the verb should be plural:

• Mięsień i ścięgno bolą. ✅
• Mięsień i ścięgno boli. ❌

Use boli only with a singular subject:

• Mięsień boli.
• Ścięgno boli.

Their plural forms are:

• mięsień → mięśnie
• ścięgno → ścięgna

Examples:

• Mięśnie bolą. = Muscles hurt.
• Ścięgna bolą. = Tendons hurt.

This is useful because the sentence already uses the plural verb bolą, so learners often want to know the actual plural nouns too.

Yes, it is grammatically correct and natural.

However, in real conversation Polish often uses slightly fuller expressions depending on context, for example:

• Boli mnie mięsień. = My muscle hurts.
• Boli mnie ścięgno. = My tendon hurts.
• Bolą mnie mięśnie. = My muscles hurt.

Your sentence, Mięsień i ścięgno bolą, is a clear neutral statement meaning that both the muscle and the tendon hurt. It sounds a bit more matter-of-fact or descriptive, which is perfectly fine.