Nie mogę się zalogować, bo sieć nie działa.

Mogę is the 1st person singular present tense form of móc, which means can / be able to.

So:

• móc = to be able to, can
• mogę = I can / I am able to

In this sentence, Nie mogę... means I can’t...

Polish often uses móc + infinitive, just like English uses can + base verb:

• Mogę wejść. = I can come in.
• Nie mogę się zalogować. = I can’t log in.

Because zalogować się is the normal Polish verb meaning to log in.

The się here is a reflexive particle, but in this case it does not literally mean oneself in the English sense. It is simply part of the verb.

So you should learn it as a whole expression:

• zalogować się = to log in
• wylogować się = to log out

Without się, the meaning changes or sounds incomplete in normal usage.

Examples:

• Muszę się zalogować. = I have to log in.
• Nie mogę się wylogować. = I can’t log out.

Both exist, but they differ in aspect, which is very important in Polish.

• logować się = imperfective
• zalogować się = perfective

Very roughly:

• logować się focuses on the process, repetition, or general activity
• zalogować się focuses on completing the action successfully

In this sentence, Nie mogę się zalogować usually means I can’t log in / I’m unable to complete the login. That is why the perfective verb is very natural here.

Compare:

• Nie mogę się zalogować. = I can’t log in.
• Nie mogę się logować. = I’m unable to log in / I can’t be logging in / I can’t log in repeatedly or in general.

In everyday speech, learners will most often hear zalogować się in this kind of situation.

In Polish, nie is normally written separately from finite verb forms.

So:

• nie mogę = I cannot
• nie działa = does not work

This is standard spelling.

Examples:

• Nie wiem. = I don’t know.
• Nie mam czasu. = I don’t have time.
• Komputer nie działa. = The computer isn’t working.

A useful beginner rule is: with verbs, nie is usually written separately.

Because Polish often drops subject pronouns when they are clear from the verb ending.

The form mogę already tells you the subject is I.

So:

• Nie mogę się zalogować = I can’t log in

You could say Ja nie mogę się zalogować, but that would usually add emphasis, for example:

• Ja nie mogę się zalogować, ale on może. = I can’t log in, but he can.

In neutral sentences, Polish usually leaves out ja.

Bo means because.

It is very common in everyday spoken and written Polish, especially in informal or neutral style.

So:

• Nie mogę się zalogować, bo sieć nie działa.
• = I can’t log in, because the network isn’t working.

You can often replace bo with ponieważ, which also means because, but ponieważ sounds a bit more formal or written.

Compare:

• bo = common, natural, conversational
• ponieważ = slightly more formal

Both are correct here.

Sieć literally means network.

In this sentence, depending on context, it could mean:

• the network
• the internet connection
• the mobile network
• the local network

So sieć nie działa means something like:

• the network isn’t working
• the internet is down
• the connection isn’t functioning

The exact English translation depends on the situation, but grammatically the Polish word is sieć.

Sieć is in the nominative singular, because it is the subject of the clause sieć nie działa.

That clause means:

• sieć = the network
• nie działa = is not working

Since sieć is the thing doing the action of working / not working, it appears in the nominative.

Also, sieć is a feminine noun.

Działa is the 3rd person singular present tense form of działać, which means to work / to function / to operate.

Because sieć is singular, the verb is also singular:

• sieć działa = the network works
• sieć nie działa = the network doesn’t work

More examples:

• Telefon działa. = The phone works.
• Internet nie działa. = The internet isn’t working.
• To nie działa. = This doesn’t work.

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

The most natural version here is:

• Nie mogę się zalogować, bo sieć nie działa.

You may also hear:

• Nie mogę zalogować się...

That is possible, but it is usually less neutral and may sound more marked or slightly less natural in ordinary speech.

A good learner rule is:

• put się early in the clause
• often after the conjugated verb

So:

• Muszę się spieszyć.
• Chcę się położyć.
• Nie mogę się zalogować.

That pattern will usually sound natural.

They mean very different things.

• Nie mogę się zalogować = I can’t log in.
• Mogę się nie zalogować = I may not log in / It’s possible that I won’t log in.

In the original sentence, nie negates mogę, so the meaning is I am not able to.

That is the normal way to say I can’t log in.

A rough guide:

• mogę ≈ MOH-gheh
  The ę at the end is nasal, but in normal speech at the end of a word it is often pronounced somewhat like a plain e with slight nasal coloring.

• się ≈ shyeng or shyeh
  Again, the final ę is nasalized, though in everyday pronunciation it may sound weaker than learners expect.

• sieć ≈ shyech
  The si before a vowel sounds like a soft sh sound, and ć is a soft consonant, something like a very soft ch/tch.

If you know IPA, a broad approximation would be:

• mogę = /ˈmɔ.ɡɛ̃/
• się = /ɕɛ̃/
• sieć = /ɕet͡ɕ/

The main thing for a learner is to remember that si before e sounds soft, not like English see.

Yes, it is completely natural.

A native speaker could definitely say:

• Nie mogę się zalogować, bo sieć nie działa.

Depending on context, they might also say:

• Nie mogę się zalogować, bo internet nie działa. = I can’t log in because the internet isn’t working.
• Nie mogę się zalogować, bo nie ma sieci. = I can’t log in because there’s no network.
• Nie mogę się zalogować, bo padła sieć. = I can’t log in because the network went down.

But your original sentence is standard, correct, and natural.

In normal usage, bo most often links two clauses, as in your sentence:

• Nie mogę się zalogować, bo sieć nie działa.

It can also appear after a pause in conversation:

• Bo sieć nie działa. = Because the network isn’t working.

That kind of sentence is common in spoken language when the first part is understood from context. But in a full neutral sentence, learners should usually use bo to connect two parts, just like in your example.