Mobilní tarif je nový.

Mobilní tarif means mobile plan or cell phone plan.

• mobilní = mobile
• tarif = tariff, rate, plan

In everyday English, mobile plan is usually the most natural translation, even though tarif is closer in form to tariff.

Je means is.

It is the 3rd person singular form of the verb být = to be.

So:

• je = is

In this sentence, it links the subject and the description:

• Mobilní tarif je nový.
• The mobile plan is new.

Because nový has to agree with tarif.

In Czech, adjectives change form to match the noun they describe. They agree in:

• gender
• number
• case

Here, tarif is:

• masculine inanimate
• singular
• nominative

So the correct adjective form is nový.

Compare:

• nový tarif = a new plan
• nová smlouva = a new contract
• nové číslo = a new number

Yes, tarif is masculine inanimate.

A useful clue is that many Czech nouns ending in a consonant are masculine. Since tarif ends in -f, that strongly suggests a masculine noun.

Because it is masculine inanimate, adjectives in the nominative singular typically take forms like:

• nový tarif
• levný tarif
• dobrý tarif

Czech does not have articles like English a, an, and the.

So Mobilní tarif je nový. can mean:

• The mobile plan is new
• A mobile plan is new

Which one sounds best depends on context. Usually the situation makes it clear.

This is very normal in Czech, so learners should get used to understanding definiteness from context rather than from articles.

Yes, Czech word order is more flexible than English.

The neutral, straightforward order is:

• Mobilní tarif je nový.

But other orders are possible, depending on emphasis:

• Nový je mobilní tarif.
• Je nový mobilní tarif.
  This can sound more marked or context-dependent.

For a beginner, the basic pattern to remember is:

• subject + je + adjective
• Mobilní tarif je nový.

That is the safest and most natural version in most situations.

In Czech, adjectives usually come before the noun they describe, just like in English.

So:

• mobilní tarif = mobile plan
• nový tarif = new plan
• levný tarif = cheap plan

That is the normal position for descriptive adjectives.

Grammatically, nový is an adjective, but in this sentence it works as part of the predicate after je.

So:

• Mobilní tarif = subject
• je nový = predicate meaning is new

This is similar to English:

• The plan is new.

The adjective is not directly attached to the noun as in nový tarif; instead, it comes after je and describes the subject.

A rough pronunciation guide is:

MO-bil-nee TA-rif ye NO-vee

More carefully:

• mobilní ≈ mo-bil-nee
• tarif ≈ ta-rif
• je ≈ ye
• nový ≈ no-vee

A few useful notes:

• í and ý are long vowels
• j is pronounced like English y in yes
• stress in Czech is usually on the first syllable of each word

So the stressed syllables are:

• MObilní
• TArif
• je
• NOvý

You may hear novej in informal spoken Czech, especially in everyday conversation, but the standard written form is nový.

So:

• nový = standard, correct, what learners should use
• novej = colloquial spoken variant

If you are learning Czech, it is best to produce nový unless you are specifically learning colloquial speech.

Yes, in this sentence they would be wrong.

Because tarif is masculine singular, the adjective must be nový.

These other forms match different noun types:

• nová = feminine singular
  example: smlouva je nová
• nové = neuter singular or some plural uses
  example: číslo je nové

So with tarif, the correct form is:

• Mobilní tarif je nový.

Normally, if this is just an ordinary sentence, Mobilní is capitalized only because it is the first word of the sentence.

If the phrase appeared in the middle of a sentence, it would usually be lowercase:

• Ten mobilní tarif je nový.

So mobilní is not capitalized because it is a special noun; it is capitalized here only because it starts the sentence.