Ona pięknie śpiewa.

• Ona = she
• pięknie = beautifully / nicely
• śpiewa = sings / is singing

Grammatically:

• ona is the subject
• pięknie is an adverb describing how she sings
• śpiewa is the verb

So the structure is basically She sings beautifully.

Because pięknie is an adverb, and here you need a word that describes the action of singing.

• piękny / piękna / piękne are adjectives: they describe a noun
• pięknie is an adverb: it describes a verb

Compare:

• Ona jest piękna = She is beautiful
  Here piękna describes ona
• Ona pięknie śpiewa = She sings beautifully
  Here pięknie describes śpiewa

English also does this:

• beautiful → adjective
• beautifully → adverb

Yes, very often.

Polish usually does not need the subject pronoun if the verb already shows who the subject is. So:

• Ona pięknie śpiewa
• Pięknie śpiewa

Both can mean She sings beautifully.

Why include ona then?

• for emphasis
• for contrast
• for clarity, if it is important to say she rather than someone else

For example:

• On źle śpiewa, ale ona pięknie śpiewa.
  He sings badly, but she sings beautifully.

Śpiewa is present tense, third person singular of śpiewać.

That means it matches:

• on = he
• ona = she
• ono = it

So:

• on śpiewa
• ona śpiewa
• ono śpiewa

In English, this can translate as:

• she sings
• she is singing

Which one is meant depends on context.

It can mean both.

Polish present tense often covers both:

• a general/habitual meaning: She sings beautifully
• a right now meaning: She is singing beautifully

Context tells you which is intended.

For example:

• Ona pięknie śpiewa w chórze. = more habitual
• Posłuchaj, ona pięknie śpiewa. = probably happening now

Yes, Polish word order is more flexible than English.

Ona pięknie śpiewa is a very natural, neutral order.

But you can also hear:

• Pięknie śpiewa.
• Ona śpiewa pięknie.
• Śpiewa pięknie.

These versions can sound slightly different in emphasis, but they are all possible.

Very roughly:

• Ona pięknie śpiewa = neutral
• Pięknie śpiewa = focuses on how she sings
• Ona śpiewa pięknie = also possible, sometimes a bit more marked or stylistic

Polish uses word order more for focus and emphasis than English does.

Because Polish has no articles.

English says:

• a singer
• the singer

Polish does not use separate words like a/an/the. The meaning comes from context.

In Ona pięknie śpiewa, there is simply no article to add.

This is normal in Polish.

A practical approximation is:

• pięk- sounds roughly like pyenk-
• -nie sounds like nye

So pięknie is approximately PYENK-nye, with the stress on the first syllable of this word itself, though in the whole sentence Polish stress normally falls on the second-to-last syllable of each word.

A few useful notes:

• pi before a vowel sounds soft, somewhat like py
• ę is a nasal vowel; before k it often sounds close to en/em to English ears
• nie here is a soft nye sound

You do not need to make it perfect immediately; being understood is more important than sounding fully native.

A helpful approximation is SHPYE-va.

More exactly:

• ś is a soft sh sound, different from plain sz
• pie sounds roughly like pye
• wa sounds like va, because Polish w is pronounced like English v

So:

• śpiewa ≈ SHPYE-va

Also remember that Polish stress is usually on the penultimate syllable, so here the stress is on śpie-:

• ŚPIE-wa

Yes:

• ś = a soft sh-like sound
• ę = a nasal vowel
• w = pronounced like English v

So in this sentence:

• ś in śpiewa is not like English s
• ę in pięknie is not like a plain English vowel
• w in śpiewa is v, not English w

These are very common Polish spelling patterns, so this sentence is good practice.

Yes. Śpiewać is imperfective.

That matters because imperfective verbs are used for:

• ongoing actions
• repeated/habitual actions
• general descriptions

That fits this sentence well: Ona pięknie śpiewa describes how she sings in general, or what she is doing now.

The perfective partner is often zaśpiewać, which usually refers to a completed singing act, for example to sing once / to sing something through / to start singing depending on context.

So:

• śpiewa = imperfective present
• zaśpiewa = perfective future form

In normal use, ona is the feminine singular pronoun, so yes, it usually means she.

Polish pronouns reflect grammatical gender:

• on = he
• ona = she
• ono = it

So Ona pięknie śpiewa clearly tells you the subject is feminine.

Polish stress is usually on the second-to-last syllable of each word.

So:

• O-na
• PIĘK-nie
• ŚPIE-wa

In connected speech, sentence emphasis can move depending on what you want to highlight, but the normal word stress stays the same.

So a neutral reading would sound roughly like:

• O-na PIĘK-nie ŚPIE-wa

Yes, depending on context.

Pięknie most directly suggests:

• beautifully
• beautiful
• sometimes more naturally in English, very well or nicely

The exact English choice depends on style:

• She sings beautifully = most direct and elegant
• She sings nicely = possible, but a bit weaker
• She sings very well = very natural in many contexts

So pięknie is stronger and more expressive than just dobrze in many situations.