Zákazník se rozhodl pro mobilní tarif.

Here se is part of the verb rozhodnout se, which means to decide.

So:

• rozhodnout = to decide something / to make a decision in some contexts
• rozhodnout se = to make up one’s mind, to decide

In this sentence, se does not literally mean himself. It is a reflexive particle that belongs to the verb.

So you should learn this as a whole expression:

• rozhodnout se pro něco = to decide on / opt for something

Rozhodnout se is the infinitive: to decide.

In the sentence, we need the past tense, because the meaning is The customer decided on a mobile plan.

The past tense of rozhodnout se here is:

• rozhodl se = decided (masculine singular)

So:

• Zákazník se rozhodl... = The customer decided...

A useful point: in Czech past tense, the form agrees with gender and number:

• rozhodl se = masculine singular
• rozhodla se = feminine singular
• rozhodlo se = neuter singular
• rozhodli se = masculine animate plural
• rozhodly se = feminine / non-masculine plural

Because zákazník is masculine singular, we get rozhodl.

In Czech, past tense is built differently from English.

With 3rd person subjects, Czech often just uses the past participle form, without an extra visible auxiliary in modern standard usage:

• Zákazník se rozhodl. = The customer decided.

In 1st and 2nd person, you do see forms of být (to be):

• Rozhodl jsem se. = I decided.
• Rozhodl ses. = You decided.

But in the 3rd person, Czech normally has no extra word here:

• on se rozhodl = he decided
• zákazník se rozhodl = the customer decided

Zákazník is in the nominative case because it is the subject of the sentence — the person doing the action.

So:

• zákazník = customer / the customer as the subject

This noun is:

• masculine animate
• singular
• nominative

That matters because the past tense form rozhodl agrees with this masculine singular subject.

After rozhodnout se, Czech often uses pro + accusative to mean:

• to decide for
• to opt for
• to choose

So:

• rozhodnout se pro něco = to decide on / opt for something

Examples:

• Rozhodl se pro auto. = He decided on a car.
• Rozhodla se pro studium medicíny. = She decided to study medicine.
• Zákazník se rozhodl pro mobilní tarif. = The customer opted for a mobile plan.

This is an idiomatic pattern, so it is best learned as a set phrase.

It is in the accusative singular because the preposition pro takes the accusative.

So:

• pro + accusative

In this phrase:

• mobilní agrees with tarif
• tarif is masculine inanimate singular

For masculine inanimate nouns, the accusative singular is often the same as the nominative singular. That is why tarif looks unchanged here.

So:

• nominative: mobilní tarif
• accusative: mobilní tarif

Same form, different function.

Because mobilní is an adjective whose accusative singular masculine inanimate form is the same as its nominative singular form here.

It agrees with tarif in:

• gender: masculine
• number: singular
• case: accusative

Since tarif is masculine inanimate, the adjective form here stays:

• mobilní tarif

You can compare it with a masculine animate noun, where accusative often changes differently:

• nominative: mladý zákazník
• accusative: mladého zákazníka

But with masculine inanimate:

• nominative: nový tarif
• accusative: nový tarif

So mobilní tarif looking unchanged is normal.

No, not in this meaning.

If you want to say The customer decided on a mobile plan, you need the reflexive verb:

• Zákazník se rozhodl pro mobilní tarif.

Without se, the sentence sounds wrong for this meaning.

That is because the natural Czech expression is:

• rozhodnout se pro něco

So it is best to memorize the whole pattern, not just the base verb.

Because rozhodnout se pro něco is the normal idiomatic construction meaning to choose / opt for something.

Different prepositions with related verbs can give different meanings. For example:

• rozhodnout o něčem = to decide about something
• rozhodnout něco = to decide something
• rozhodnout se pro něco = to decide on / choose something

So in your sentence, the meaning is specifically to opt for a mobile plan, which is why pro is used.

The sentence has a neutral, natural word order:

• Zákazník se rozhodl pro mobilní tarif.

Czech word order is fairly flexible, but changing it usually changes the emphasis.

For example:

• Pro mobilní tarif se rozhodl zákazník.
  This puts more emphasis on for a mobile plan or contrasts the customer with someone else.

• Zákazník pro mobilní tarif se rozhodl.
  Grammatically possible in some contexts, but less natural as a neutral sentence.

So the given version is the best neutral choice.

Czech has no articles like English a/an and the.

So:

• zákazník can mean a customer or the customer
• mobilní tarif can mean a mobile plan or the mobile plan

Which one is meant depends on context.

That means the sentence could be translated as:

• The customer decided on a mobile plan.
• A customer decided on the mobile plan.
• The customer opted for the mobile tariff.

The exact English article choice depends on the situation.

The hardest part for many learners is ř in rozhodl.

Very roughly:

• rozhodl sounds something like rzhodl, with a special Czech sound ř
• ř is a unique Czech consonant, something like a blend of r and zh sound

A rough pronunciation guide for the whole sentence:

• Zákazník se rozhodl pro mobilní tarif.
• ZAA-kaz-neek se roz-hodl pro mo-beel-nee ta-rif

A few helpful notes:

• Czech stress is usually on the first syllable of each word:
  • ZÁkazník
  • ROZhodl
  • MObilní
  • TArif

If ř is difficult, that is completely normal. It is one of the most challenging Czech sounds for English speakers.

The dictionary forms are:

• zákazník = customer
• se = reflexive particle
• rozhodnout se = to decide
• pro = for
• mobilní = mobile
• tarif = plan / tariff

So the core pattern to learn is:

• rozhodnout se pro + accusative = to decide on / opt for

That pattern will help you build many similar sentences.