Sygnał jest za słaby, więc nie mogę oddzwonić.

Because sygnał is the subject of the sentence, so it stays in the nominative case.

• sygnał = signal
• jest za słaby = is too weak

So the structure is simply:

• Sygnał = the signal
• jest = is
• za słaby = too weak

If sygnał had a different role in the sentence, its form might change, but here it is the thing being described.

Jest is the 3rd person singular form of być (to be) in the present tense.

So:

• sygnał jest za słaby = the signal is too weak

In Polish, you often omit subject pronouns like on / ona / ono, but you do not usually omit the verb jest in a normal sentence like this.

Za placed before an adjective usually means too.

So:

• słaby = weak
• za słaby = too weak

Other similar examples:

• za drogi = too expensive
• za trudny = too difficult
• za mały = too small

So jest za słaby literally means is too weak.

Because the adjective has to agree with the noun it describes.

Here the noun is sygnał, which is:

• masculine
• singular
• nominative

So the adjective also takes the masculine singular nominative form:

• słaby

Compare:

• telefon jest za drogi → masculine
• sieć jest za słaba → feminine
• połączenie jest za słabe → neuter

Więc means so, therefore, or thus. It introduces a result.

• Sygnał jest za słaby, więc nie mogę oddzwonić. = The signal is too weak, so I can’t call back.

By contrast, bo means because and introduces a reason.

Compare:

• Nie mogę oddzwonić, bo sygnał jest za słaby. = I can’t call back because the signal is too weak.

So:

• więc = result
• bo = reason

In Polish, a comma is normally used before conjunctions like więc, bo, ale, że, when they connect clauses.

So the comma in:

• Sygnał jest za słaby, więc nie mogę oddzwonić.

is standard punctuation.

This is similar to English, where a comma often appears before so when linking two full clauses.

Mogę means I can / I am able to, and after móc (can, to be able to) Polish uses the infinitive of the main verb.

So:

• mogę oddzwonić = I can call back
• nie mogę oddzwonić = I can’t call back

This is very similar to English structure:

• I can call back

except that in Polish the infinitive is one word: oddzwonić.

Oddzwonić means to call back or to return a phone call.

It is built from:

• dzwonić = to call / to ring
• od- = a prefix that often adds the idea of back, in return, or as a response

So:

• zadzwonić = to call
• oddzwonić = to call back

This verb is specifically about returning a call, not just making any call.

Because oddzwonić specifically means to call back, not simply to call.

Compare:

• Nie mogę dzwonić. = I can’t call.
• Nie mogę oddzwonić. = I can’t call back.

If the intended meaning is that someone contacted you first and you want to return the call, oddzwonić is the natural choice.

Oddzwonić is perfective. It refers to a completed action: making that return call successfully.

That works well here because the sentence is about being unable to complete a specific action:

• nie mogę oddzwonić = I can’t call back

The imperfective partner is usually oddzwaniać, which would suggest repeated, ongoing, or habitual calling back.

Compare:

• Nie mogę oddzwonić. = I can’t call back (complete the return call)
• Nie mogę oddzwaniać co pięć minut. = I can’t keep calling back every five minutes

So the perfective verb is the natural choice in this sentence.

Yes. In everyday Polish, sygnał in this kind of sentence commonly refers to phone signal, mobile reception, or network strength.

So Sygnał jest za słaby is a very natural way to say that reception is too weak to make or return a call.

Depending on context, Polish speakers might also say things like:

• Nie mam zasięgu. = I have no reception / no signal.
• Mam słaby zasięg. = I have weak reception.

But sygnał jest za słaby is still understandable and natural.

Yes, Polish word order is fairly flexible, although some versions sound more natural than others.

The original:

• Sygnał jest za słaby, więc nie mogę oddzwonić.

is clear and natural.

You could also hear:

• Nie mogę oddzwonić, bo sygnał jest za słaby.
• Sygnał jest za słaby, dlatego nie mogę oddzwonić.

Changing the order can slightly shift the emphasis, but the original version is completely normal.

Because Polish usually does not need subject pronouns when the verb form already shows who the subject is.

• mogę = I can

So nie mogę oddzwonić already means I can’t call back without needing ja.

You can add ja for emphasis:

• Ja nie mogę oddzwonić. = I can’t call back.

But without emphasis, leaving it out is more natural.

You would change the tense of móc and possibly the context.

Past:

• Sygnał był za słaby, więc nie mogłem oddzwonić. = The signal was too weak, so I couldn’t call back.

If the speaker is female:

• nie mogłam oddzwonić

Future:

• Jeśli sygnał będzie za słaby, nie będę mógł oddzwonić. = If the signal is too weak, I won’t be able to call back.

Again, for a female speaker:

• nie będę mogła oddzwonić

A rough pronunciation is:

• oddzwonić ≈ od-dzvo-nyeech

A few helpful notes:

• dzw can feel difficult for English speakers; it is pronounced as one cluster, roughly like dzv
• o is like o in for but shorter and cleaner
• ń in -nić is a soft n, like the ny sound in canyon
• the stress is normally on the second-to-last syllable:
  • od-dzwo-NIĆ

And for the whole sentence, stress follows normal Polish patterns, usually penultimate in each word:

• SYG-nał jest za SŁA-by, WIĘC nie MO-gę od-dzwo-NIĆ

Yes, but it changes the meaning.

• nie mogę zadzwonić = I can’t call
• nie mogę oddzwonić = I can’t call back

So if you simply mean you are unable to make a call, zadzwonić works. If you mean you are returning someone’s call, oddzwonić is better.