Musím večer nabít telefon, protože zítra půjdu na poštu.

In Czech, subject pronouns are often left out when they are clear from the verb ending.

• musím = I must / I have to
• the ending -ím already tells you the subject is I

So Musím večer nabít telefon naturally means I have to charge my phone in the evening / tonight without needing já.

You could say Já musím..., but that usually adds emphasis, like I have to.

Večer here means in the evening / tonight.

In Czech, some time expressions are used without a preposition, and večer is one of them.

So:

• večer = in the evening / tonight
• ráno = in the morning
• zítra = tomorrow

You do not say v večer in standard Czech for this meaning.

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

• nabít = perfective → to charge up, to complete the charging
• nabíjet = imperfective → to be charging / to charge repeatedly / in a general sense

In this sentence, the speaker means a single completed action: they need to make sure the phone gets charged before tomorrow. So nabít is the natural choice.

Compare:

• Musím nabít telefon. = I need to charge the phone up.
• Musím nabíjet telefon každý den. = I have to charge the phone every day.

Telefon is the direct object, so it is in the accusative case.

But for many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: telefon
• accusative: telefon

That is why you do not see a different ending here.

Protože means because.

It introduces the reason:

• Musím večer nabít telefon = I have to charge my phone tonight
• protože zítra půjdu na poštu = because tomorrow I’ll go to the post office

So the second clause explains why the speaker needs to charge the phone.

In Czech, a comma is normally written before conjunctions that introduce a subordinate clause, including protože.

So:

• Musím večer nabít telefon, protože zítra půjdu na poštu.

This is standard Czech punctuation.

Půjdu is future tense: I will go.

It comes from the verb jít = to go.

• jdu = I am going / I go
• půjdu = I will go

This verb is irregular, so you just have to learn the future forms:

• půjdu = I will go
• půjdeš = you will go
• půjde = he/she/it will go
• etc.

Because jít has its own special future form.

In Czech, many motion verbs form the future without using budu.

So:

• půjdu = I will go

You do not normally say budu jít for this meaning.

By contrast, with many imperfective verbs you do use budu:

• budu dělat = I will do
• budu čekat = I will wait

But:

• půjdu = I will go

This is a very common learner question.

In Czech, some destinations are expressed with na + accusative, especially places understood as institutions, services, events, or activities.

So:

• jít na poštu = to go to the post office
• jít na policii = to go to the police
• jít na úřad = to go to the office/authority
• jít na koncert = to go to a concert

Using do pošty would usually not be the normal choice here.

Because na with movement usually takes the accusative case.

The base form is:

• pošta = post office / mail

Accusative singular:

• poštu

So:

• na poštu = to the post office

This is a regular pattern for many feminine nouns ending in -a:

• škola → školu
• práce is different because it has a different declension pattern
• pošta → poštu

Yes. Czech word order is quite flexible, because endings carry much of the grammar.

The original sentence is natural:

• Musím večer nabít telefon, protože zítra půjdu na poštu.

But other orders are also possible, depending on emphasis:

• Večer musím nabít telefon, protože zítra půjdu na poštu.
• Zítra půjdu na poštu, takže musím večer nabít telefon.

The original version sounds neutral and natural.

It can often mean either, depending on context.

In this sentence, since the next clause says zítra (tomorrow), večer will usually be understood as this evening / tonight.

So the idea is:

• I need to charge my phone tonight, because tomorrow I’m going to the post office.

It often implies both the place and the purpose.

Jít na poštu usually means to go to the post office to handle post-related business—for example:

• send a parcel
• pick something up
• deal with mail

So it is not just a bare physical destination; it often carries the practical meaning of going there for post office business.

The accent mark in Czech usually shows vowel length, not stress.

So:

• nabít has a long í
• it is pronounced roughly like na-BEET

A very important point:

• i = short vowel
• í = long vowel

Czech stress is usually on the first syllable of the word, so NAbít, but the í is still long.

Yes, telefon is perfectly normal and common.

Depending on context, Czech speakers may also say:

• mobil = mobile phone / cell phone
• mobilní telefon = mobile phone

So you could also hear:

• Musím večer nabít mobil...

That may sound even more natural in everyday speech if you specifically mean a cell phone.