Opłata stała jest niska.

It means fixed fee, flat fee, or in some contexts standing charge.

• opłata = fee, charge, payment
• stała = fixed, constant

So Opłata stała jest niska. means that the fixed part of a charge is low.

Because stała has to agree with opłata.

In Polish, adjectives change their form to match the noun in:

• gender
• number
• case

Here:

• opłata is feminine singular
• so the adjective must also be feminine singular
• therefore: stała

If the noun were masculine, you might get stały instead.

For the same reason: adjective agreement.

niska agrees with opłata, which is feminine singular.

• niski = masculine singular
• niska = feminine singular
• niskie = neuter singular or some plural forms

Since the subject is opłata stała, the describing adjective must be niska.

It is in the nominative case because it is the subject of the sentence.

The structure is:

• Opłata stała = the fixed fee
• jest = is
• niska = low

So the sentence literally works like The fixed fee is low.

In normal standard Polish, jest is the present-tense form of to be and is usually included here.

So:

• Opłata stała jest niska. = normal, complete sentence

You may sometimes see jest omitted in headings, notes, ads, or very compressed style:

• Opłata stała niska — this would sound incomplete in normal prose

So for a learner, the safe standard version is definitely:

• Opłata stała jest niska.

No. Polish does not use articles like the and a/an.

So opłata stała can mean:

• the fixed fee
• a fixed fee

Which one sounds best depends on context. In many real situations, English would naturally translate it as the fixed fee.

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

The neutral, straightforward order is:

• Opłata stała jest niska.

But other orders are possible for emphasis, for example:

• Niska jest opłata stała. — emphasizes low
• Jest niska opłata stała. — less neutral, more marked

For everyday learning, use Opłata stała jest niska. as the most natural basic version.

A rough English-friendly guide:

• Opłata ≈ oh-PWA-ta
• stała ≈ STA-wa
• jest ≈ yest
• niska ≈ NEES-ka

A few important sounds:

• ł sounds like English w
• j sounds like English y
• a is a clear ah sound

So the whole sentence is roughly: oh-PWA-ta STA-wa yest NEES-ka

Not always. In this sentence, yes: it is an adjective meaning fixed or constant.

But stała can also be a noun in some contexts, for example in mathematics or science, where it can mean constant.

Here, though, it clearly modifies opłata, so it is an adjective:

• opłata stała = fixed fee

Yes, very often.

In practical contexts, opłata stała is commonly used for a fixed charge that does not depend on usage, for example:

• electricity bills
• gas bills
• phone services
• administrative fees

So depending on context, a very natural translation could be:

• The standing charge is low.
• The fixed fee is low.

Because in this sentence, niska is a predicate adjective describing the subject, and it stays in the form that agrees with the subject.

So:

• opłata = feminine singular nominative
• niska matches it as feminine singular

This is similar to:

• Kawa jest dobra. = The coffee is good.
• Cena jest wysoka. = The price is high.

The adjective after jest still agrees with the subject it describes.