Min arm gör ont, men mitt ben känns bättre idag.

Because Swedish nouns have grammatical gender, and the possessive word my changes to match it.

• arm is a common gender noun: en arm
• ben is a neuter gender noun: ett ben

So:

• min arm = my arm
• mitt ben = my leg

A quick pattern:

• min
  • en-word
• mitt
  • ett-word
• mina
  • plural nouns

Examples:

• min hand
• mitt huvud
• mina händer

In Swedish, you normally do not use the definite article together with a possessive in this kind of phrase.

So Swedish says:

• min arm = my arm
• not min armen

This is similar to English, where we say my arm, not my the arm.

Gör ont is a very common Swedish expression meaning hurts or causes pain.

Literally, gör means does/makes, and ont means painfully / in a painful way. But you should learn gör ont as a set phrase:

• Min arm gör ont. = My arm hurts.
• Det gör ont. = It hurts.

Swedish often uses this expression where English simply uses hurt.

In this sentence, ont is usually best understood as an adverb-like form used in fixed expressions about pain.

You will often see it in phrases like:

• göra ont = to hurt
• ha ont = to be in pain / to have pain

Examples:

• Jag har ont i ryggen. = My back hurts / I have pain in my back.
• Det gör ont i benet. = The leg hurts / It hurts in the leg.

So even though English learners may want to treat ont like a normal adjective, it is better to learn it as part of these common pain expressions.

Känns comes from the verb kännas, which means to feel in the sense of seem or feel like.

So:

• mitt ben känns bättre = my leg feels better

This does not mean that the leg is doing the feeling itself in a human sense. It means that the condition of the leg feels improved.

Compare:

• Jag känner mig bättre. = I feel better.
• Mitt ben känns bättre. = My leg feels better.

The first is about I feeling better overall.
The second is about the leg seeming or feeling better.

The dictionary form is kännas. In the present tense, it becomes känns.

This is just how that verb works:

• infinitive: kännas
• present: känns
• past: kändes

Examples:

• Det känns bra. = It feels good.
• Armen känns stel. = The arm feels stiff.

Even though -s sometimes marks a passive in Swedish, here you should simply learn kännas as its own verb meaning to feel / seem.

Both can be possible in some contexts, but känns bättre is more natural when talking about how a body part is doing from the speaker’s point of view.

• Mitt ben känns bättre = My leg feels better
• Mitt ben är bättre can sound more direct or clinical, like my leg is better

When talking about symptoms, pain, recovery, or comfort, Swedish very often uses känns.

Yes, absolutely. Both are natural, but they focus slightly differently.

• Min arm gör ont. = My arm hurts.
• Jag har ont i armen. = I have pain in my arm / My arm hurts.

The second structure is extremely common in Swedish when talking about pain in a body part.

A helpful pattern:

• Jag har ont i huvudet. = I have a headache / My head hurts.
• Hon har ont i ryggen. = Her back hurts.

So learners should know both patterns:

• [body part] gör ont
• ha ont i [body part]

Swedish often places time expressions like idag at the end of a clause, especially when it sounds natural there.

So:

• mitt ben känns bättre idag

is a very normal word order.

You could sometimes move idag earlier for emphasis, but the end position is neutral and common.

Compare:

• Mitt ben känns bättre idag.
• Idag känns mitt ben bättre.

Both are possible, but the second gives more emphasis to today.

Because bättre is the comparative form of bra (good / well), and in this use it does not change for gender or number.

• en arm känns bättre
• ett ben känns bättre
• mina händer känns bättre

It stays bättre in all of these.

Useful comparison:

• bra = good/well
• bättre = better
• bäst = best

Men means but and connects the two clauses:

• Min arm gör ont
• men mitt ben känns bättre idag

After men, Swedish keeps normal main-clause word order. That means the subject still comes before the verb:

• mitt ben känns bättre idag

This is important because some other Swedish connecting words can change the word order, but men does not.

In the plural, the possessive becomes mina.

Examples:

• Mina armar gör ont. = My arms hurt.
• Mina ben känns bättre idag. = My legs feel better today.

So the pattern is:

• min for en-words
• mitt for ett-words
• mina for plurals

Yes, ben can mean both leg and bone, depending on context.

In a sentence like:

• mitt ben känns bättre idag

the natural meaning is usually my leg feels better today.

If the context were medical or anatomical, it could mean bone, but everyday context usually makes the meaning clear. This is a normal Swedish ambiguity, and native speakers rely on context to understand it.