On źle mówi po polsku.

Yes. Polish often drops subject pronouns because the verb ending usually shows who the subject is.

So both of these are possible:

• On źle mówi po polsku.
• Źle mówi po polsku.

Both can mean He speaks Polish badly.

Including on adds emphasis, contrast, or clarity. For example, you might say On źle mówi po polsku, ale jego siostra mówi świetnie — He speaks Polish badly, but his sister speaks excellently.

Because źle is an adverb, and it modifies the verb mówi (speaks).

• zły = bad, evil, wrong — an adjective
• źle = badly, poorly, wrong — an adverb

Since the sentence is describing how he speaks, Polish uses the adverb:

• On źle mówi... = He speaks badly...

Compare:

• To jest zły nauczyciel. = This is a bad teacher.
• On źle mówi po polsku. = He speaks Polish badly.

Polish commonly expresses languages with the pattern:

• mówić po + language adverb

So:

• po polsku = in Polish
• po angielsku = in English
• po niemiecku = in German

That is why mówi po polsku means speaks Polish or more literally speaks in Polish.

This is one of the standard ways to talk about speaking a language in Polish.

It is historically connected with the locative case after po.

You do not need to overanalyze it at first, but it helps to recognize the pattern:

• po polsku
• po angielsku
• po francusku

These forms are fixed, very common expressions meaning in Polish, in English, in French, etc.

For learners, it is best to memorize po + language form as a chunk.

Usually, no, not in standard Polish for this meaning.

To say someone speaks a language, Polish most naturally uses:

• mówi po polsku
• mówi po angielsku

Saying mówi polski sounds unnatural in standard Polish.

However, with other verbs, you may see the language as a noun object:

• zna polski = he knows Polish
• uczy się polskiego = he is learning Polish

So with mówić, the normal pattern is po polsku, not polski.

Mówi is the 3rd person singular form of mówić, which usually means:

• to speak
• to say
• to talk

In this sentence, because of po polsku, it clearly means speaks:

• On źle mówi po polsku. = He speaks Polish badly.

Without context, mówi could also mean says in another sentence:

• On mówi prawdę. = He tells the truth / He is speaking the truth.

Yes, Polish word order is more flexible than English word order.

The neutral order here is:

• On źle mówi po polsku.

But other orders are possible for emphasis, for example:

• On mówi źle po polsku.
• Po polsku on mówi źle.
• Źle mówi po polsku.

These do not all sound equally neutral. The original sentence is the most straightforward and natural for a learner.

In Polish, changing word order often changes focus rather than basic meaning.

Źle is often tricky for English speakers.

A rough guide:

• ź sounds somewhat like a soft zh sound
• źle is approximately like zh-leh, but softer than English zh

Important points:

• The ź is soft/palatalized.
• The l in źle is a normal Polish l, not English dark l.
• The word is one syllable cluster at the start: źle.

It may help to listen to native audio and repeat slowly.

They are very close in meaning, but not equally common in everyday speech.

• po polsku = in Polish
• w języku polskim = in the Polish language

In normal conversation, po polsku is much more natural and common.

Examples:

• Mówi po polsku. = natural everyday Polish
• Mówi w języku polskim. = correct, but more formal or heavy

It means he speaks Polish badly, not necessarily that he is bad at speaking in general.

The phrase po polsku limits the meaning to that language.

So it suggests:

• his Polish is poor
• he makes mistakes in Polish
• he is not good at speaking Polish

It does not automatically mean he is bad at communication overall.

Yes. Źle can mean:

• badly / poorly
• wrongly / incorrectly

In this sentence, both ideas may overlap:

• On źle mówi po polsku.

This can suggest that his Polish is poor in quality, or that he says things incorrectly in Polish. In practice, the sentence usually means He speaks Polish badly / poorly.

You change the subject and the verb form:

• Ona źle mówi po polsku. = She speaks Polish badly.
• Oni źle mówią po polsku. = They (masculine personal/mixed group) speak Polish badly.
• One źle mówią po polsku. = They (non-masculine-personal group) speak Polish badly.

Notice:

• mówi = singular
• mówią = plural

Usually it expresses general ability or habitual behavior:

• On źle mówi po polsku. = He speaks Polish badly.

It normally means that his Polish-speaking ability is poor.

Depending on context, Polish present tense can also describe what is happening now, but this sentence is most naturally understood as a general statement about his level of Polish.