Po chorobie nadal jestem słaby.

Because the preposition po changes the form of the noun that follows it. In this sentence, po means after, and it normally requires the locative case.

So:

• dictionary form: choroba = illness
• after po: po chorobie = after the illness / after an illness

This is a very common pattern in Polish:

• po pracy = after work
• po obiedzie = after dinner
• po lekcji = after the lesson

The basic dictionary form is choroba.

Here is the change:

• choroba → nominative singular
• chorobie → locative singular

So if you look the word up in a dictionary, search for choroba, not chorobie.

Nadal means still or continuing to be.

So Po chorobie nadal jestem słaby means that the weakness has not gone away yet.

A close alternative is wciąż:

• Po chorobie wciąż jestem słaby = After the illness, I’m still weak.

You may also hear jeszcze in similar sentences, but nadal is often clearer because it strongly focuses on continuation.

Polish often leaves out subject pronouns when they are not needed. The verb form already tells you who the subject is.

Here, jestem means I am, so ja is unnecessary.

• Jestem słaby = I am weak
• Ja jestem słaby = I am weak, with extra emphasis on I

You would usually add ja only for contrast or emphasis:

• Ja nadal jestem słaby, ale ona już czuje się dobrze. = I’m still weak, but she already feels well.

Słaby is the masculine singular form of the adjective weak. Polish adjectives must agree with the person they describe.

So:

• a man says: jestem słaby
• a woman says: jestem słaba

That means the full sentence becomes:

• Po chorobie nadal jestem słaby — said by a man
• Po chorobie nadal jestem słaba — said by a woman

Because słaby is an adjective, and here it describes the person: I am weak.

Słabo is usually an adverb, and it is used in different structures. For example:

• Jestem słaby = I am weak
• Jest mi słabo = I feel faint / I feel weak / I feel unwell

So słaby describes me, while słabo describes how I feel in a more adverb-like way.

After być (to be), adjectives normally appear in the nominative and agree with the subject.

So:

• jestem słaby
• jestem zmęczony
• jestem chory

This is different from predicate nouns, which are often in the instrumental:

• jestem nauczycielem = I am a teacher

So here, because słaby is an adjective, its form is exactly what you would expect.

Yes. Polish word order is fairly flexible, because endings show grammatical relationships.

The version you have is very natural:

• Po chorobie nadal jestem słaby.

But these are also possible:

• Nadal jestem słaby po chorobie.
• Po chorobie jestem nadal słaby.
• Jestem nadal słaby po chorobie.

They all mean roughly the same thing, but the emphasis shifts a little. Starting with Po chorobie gives the time/background first: after the illness...

Polish has no articles, so there is no separate word for a, an, or the.

That means po chorobie can mean:

• after the illness
• after an illness
• after being ill

The exact meaning depends on context. In real conversation, listeners usually understand it without any problem.

A rough pronunciation is:

po kho-RO-byeh NA-dal YES-tem SWA-bih

A few helpful notes:

• ch sounds like the ch in Scottish loch or German Bach
• ł sounds like English w
• ó sounds like u
• Polish stress is usually on the second-to-last syllable

So:

• chorobie → cho-RO-bie
• nadal → NA-dal
• jestem → JES-tem
• słaby → SŁA-by