Můj telefon teď nejde, proto ti nemůžu zavolat.

Because telefon is a masculine inanimate singular noun, and the possessive pronoun můj has to agree with it.

Here the forms are:

• můj = my, for masculine singular nouns
• moje / má = my, for feminine singular or neuter plural depending on form
• moji / mé etc. = other forms depending on case/number/style

So:

• můj telefon = my phone
• moje kniha = my book
• moje auto = my car

The pronoun changes form just like an adjective would.

Teď means now / right now / at the moment.

In this sentence, it tells us that the problem is temporary or current:

• Můj telefon teď nejde = My phone isn’t working right now

Its position is natural here, after telefon, but Czech word order is fairly flexible. You could also hear:

• Teď můj telefon nejde
• Můj telefon nejde teď

But the original sentence sounds the most natural in ordinary speech.

Yes, jít literally means to go, but in Czech it is also commonly used for things like machines, devices, programs, or systems to mean to work / to function / to run.

So:

• Telefon nejde = The phone doesn’t work
• Počítač nejde = The computer isn’t working
• Internet nejde = The internet is down / not working

Here nejde is:

• ne- = negation
• jde = 3rd person singular present of jít

So literally it looks like is not going, but idiomatically it means isn’t working.

Yes, absolutely.

• Můj telefon teď nejde = My phone isn’t working right now
• Můj telefon teď nefunguje = My phone isn’t functioning right now

Both are natural. The difference is mostly one of style and feel:

• nejde is very common in everyday speech
• nefunguje is a bit more literal and explicit

For a learner, it is useful to know both.

Proto means therefore, so, or that’s why.

It connects the two parts of the sentence:

• Můj telefon teď nejde = My phone isn’t working right now
• proto ti nemůžu zavolat = therefore / so I can’t call you

A very natural English equivalent is often so.

Czech also has the expression protože, but that means because, not therefore:

• proto = therefore / so
• protože = because

That distinction is very important.

Because the sentence contains two clauses:

• Můj telefon teď nejde
• proto ti nemůžu zavolat

In Czech, clauses are usually separated by a comma, especially when they are linked by a connecting word like proto.

So the comma is normal and expected here.

Ti is the dative form of ty and means to you.

That is exactly what you need with zavolat, because in Czech you call to someone using the dative:

• zavolat komu? = to call whom?
• ti = to you

By contrast, tě is accusative, used for direct objects in other situations, but not here.

Compare:

• Zavolám ti. = I’ll call you.
• Vidím tě. = I see you.

So with zavolat, Czech uses the dative pronoun.

Both are possible, but they are used differently.

• ti is the normal unstressed clitic form
• tobě is the stressed/full form, used for emphasis, contrast, or after certain prepositions

So:

• Nemůžu ti zavolat. = neutral, normal
• Nemůžu zavolat tobě. = I can’t call you (maybe but I can call someone else)
• Tobě nemůžu zavolat. = stronger emphasis

In everyday neutral sentences, Czech usually prefers the short clitic form ti.

Because Czech often uses a modal verb + infinitive, just like English:

• můžu = I can
• nemůžu = I can’t
• zavolat = to call

So:

• nemůžu zavolat = I can’t call

This works very much like English can’t call.

Also, nemůžu is a common spoken form of nemohu:

• nemůžu = common, conversational
• nemohu = more formal or literary

Both are correct.

This is about aspect, which is very important in Czech.

• volat = imperfective
• zavolat = perfective

In this sentence, zavolat is used because the speaker means to make a call / to give you a call, seen as a single complete action.

So:

• Nemůžu ti zavolat. = I can’t call you / give you a call
• Nemůžu ti volat. = I can’t be calling you / I can’t call you in an ongoing or repeated sense

With phone calls, zavolat is often the most natural choice when referring to one completed call.

It is standard and very common, especially in spoken Czech.

The full formal form is:

• nemohu = I cannot

The everyday spoken form is:

• nemůžu = I can’t

Both are correct, but nemůžu is what you will hear very often in real conversation.

Similarly:

• mohu / můžu
• nemohu / nemůžu

Yes, but Czech word order is more flexible than English, especially because short pronouns like ti tend to go in a special early position.

In the clause:

• proto ti nemůžu zavolat

the short pronoun ti naturally comes early, usually after the first element of the clause.

This is part of Czech clitic placement. So the order sounds natural because:

• proto comes first
• ti comes right after it
• then nemůžu zavolat

You might also hear other word orders in special contexts, but the original is very natural and idiomatic.