الكرة الطبية صغيرة لكنها نظيفة.

Yes, literally it is the medical ball:

• الكرة = the ball
• الطبية = medical

So the phrase is a noun plus an adjective: the medical ball.

In natural English, though, this kind of object is usually called a medicine ball, so that is often the best translation.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• الكرة الطبية = the medical ball
• literally: the ball medical

This is the normal word order in Arabic for noun + adjective combinations.

Because Arabic adjectives agree with the noun they describe in definiteness as well as gender and number.

Here:

• الكرة = the ball (definite)
• الطبية = the medical (also definite, to match)

So:

• كرة طبية = a medical ball
• الكرة الطبية = the medical ball

In Arabic, if the noun is definite, the adjective is usually definite too.

Because صغيرة and نظيفة are not being used the same way as الطبية.

There are two different jobs here:

1. الطبية is an adjective inside the noun phrase

   • الكرة الطبية = the medical ball

2. صغيرة and نظيفة are predicates

   • The medical ball is small
   • it is clean

So صغيرة and نظيفة mean small and clean as statements about the ball, not as part of its name.

That is why:

• الكرة الطبية = the medical ball
• صغيرة = is small
• نظيفة = is clean

Because كرة is a feminine noun in Arabic, and words that describe it must agree with it.

So the sentence uses feminine singular forms:

• الطبية = feminine singular
• صغيرة = feminine singular
• نظيفة = feminine singular

This is grammatical gender, not biological sex. Many inanimate nouns in Arabic are grammatically masculine or feminine, and كرة happens to be feminine.

In present-tense Arabic nominal sentences, the verb to be is usually not written.

So:

• الكرة الطبية صغيرة literally looks like the medical ball small
• but it means the medical ball is small

This is completely normal in Arabic.

If you wanted a past-tense version, then a form of كان could appear, for example:

• كانت الكرة الطبية صغيرة = the medical ball was small

لكنها means but it or however, it.

It is made of:

• لكن / لكنّ = but / however
• ها = it / her (feminine singular pronoun)

So:

• لكنها نظيفة = but it is clean

In fully marked Arabic, this is often understood as لكنَّها, but in normal writing the extra vowel marks are usually omitted.

Because ها is the pronoun for a feminine singular noun, and كرة is feminine.

So Arabic says, in effect:

• الكرة = feminine noun
• therefore it must also be feminine
• so but it becomes لكنها

If the noun were masculine, Arabic would use a masculine pronoun instead.

A fully vocalized version would be:

الكرةُ الطبيةُ صغيرةٌ لكنها نظيفةٌ

Those endings show that:

• الكرةُ is nominative
• الطبيةُ matches it
• صغيرةٌ is the predicate
• نظيفةٌ is also a predicate after لكنها

In everyday Arabic writing, these short vowel endings are usually omitted, so learners often see:

الكرة الطبية صغيرة لكنها نظيفة.

Both represent the same sentence; the second is just the normal unvowelled spelling.