Předplacená karta není tak drahá jako mobilní tarif.

Předplacená is an adjective-like form meaning prepaid. It comes from the verb předplatit / předplácet, meaning to pay for in advance / to subscribe.

So in předplacená karta, it describes the noun karta and works just like an adjective in the sentence.

A useful way to think of it:

• před- = in advance / pre-
• platit = to pay
• předplacená = paid for in advance, so prepaid

Because they both agree with karta, which is:

• feminine
• singular
• nominative here

In Czech, adjectives must agree with the noun they describe in gender, number, and case.

So:

• předplacená karta
• drahá karta

Both adjectives take the feminine singular nominative ending -á.

Compare:

• drahý tarif = masculine singular
• drahá karta = feminine singular
• drahé předplatné = neuter singular

Because mobilní is a soft adjective, and soft adjectives follow a different pattern from adjectives like drahý/drahá/drahé.

The adjective mobilní often keeps the same basic form in the nominative singular across genders:

• mobilní tarif = masculine
• mobilní karta = feminine
• mobilní připojení = neuter

So the form mobilní is already correct here. It does decline in other cases, but not in a way that looks like drahá.

Není means is not. It is the 3rd person singular present form of být (to be) in the negative.

Compare:

• je = is
• není = is not

So:

• Karta je drahá. = The card is expensive.
• Karta není drahá. = The card is not expensive.

Here, the sentence is making a negative comparison:

• není tak drahá jako... = is not as expensive as...

Tak ... jako is the standard Czech pattern for as ... as comparisons.

The structure is:

X je/není tak + adjective + jako Y

So here:

• není tak drahá jako mobilní tarif
• literally: is not so expensive as a mobile plan
• natural English: is not as expensive as a mobile plan

Examples:

• Je tak vysoký jako já. = He is as tall as I am.
• Není tak rychlá jako vlak. = It is not as fast as a train.

A close alternative is stejně ... jako, for example:

• není stejně drahá jako mobilní tarif

But tak ... jako is very common and natural.

Because tak drahá jako and dražší než express two different comparison patterns.

1. tak drahá jako = as expensive as
2. dražší než = more expensive than

So:

• Předplacená karta není tak drahá jako mobilní tarif. = The prepaid card is not as expensive as the mobile plan.

A more direct alternative would be:

• Předplacená karta je levnější než mobilní tarif. = The prepaid card is cheaper than the mobile plan.

Both are natural, but they are framed differently:

• není tak drahá jako = softer, less direct
• je levnější než = more direct

In this kind of comparison, jako usually introduces the thing being compared, and the noun often appears in its basic nominative-style form.

So here:

• mobilní tarif is understood as nominative

A helpful way to think about it is that there is an implied full clause behind the comparison, something like:

• ... jako je mobilní tarif
• ... as a mobile plan is

Also, in this specific phrase, the form does not visibly help much, because:

• tarif has the same form in nominative and accusative
• mobilní also looks the same here

But the normal comparison pattern is still:

• tak drahá jako mobilní tarif not something like
• tak drahá jako mobilní tarifem

Because Czech does not have articles like English a/an/the.

So a noun like karta can mean:

• a card
• the card

The exact meaning depends on context.

That is normal in Czech. The sentence does not need any article word to be grammatical.

Not completely. Czech word order is more flexible than English word order.

This sentence has a very neutral, natural order:

Předplacená karta není tak drahá jako mobilní tarif.

That is basically:

• topic first: předplacená karta
• then the verb: není
• then the comparison

You can change the order for emphasis, but the meaning or focus may shift.

For example:

• Mobilní tarif není tak drahý jako předplacená karta.

This is also grammatical, but now the subject is mobilní tarif, so the sentence means something different.

So the given word order is the normal one for this meaning.

Yes, absolutely. That is a very natural alternative.

Compare:

• Předplacená karta není tak drahá jako mobilní tarif.
• Předplacená karta je levnější než mobilní tarif.

They are very close in meaning.

The difference is mostly in style:

• není tak drahá jako = not as expensive as
  
  • slightly less direct
• je levnější než = is cheaper than
  
  • more direct

English works the same way:

• It’s not as expensive as...
• It’s cheaper than...

Czech stress is usually on the first syllable of each word.

So you would stress:

• PŘED-placená
• KAR-ta
• NE-ní
• TAK
• DRA-há
• JA-ko
• MO-bilní
• TA-rif

A few pronunciation notes:

• á and í are long vowels
• the hardest sound here for English speakers is ř in předplacená
• if ř is difficult, an approximate learner version is fine at first; it is better to say the word clearly than to over-focus on perfection

So a rough pronunciation guide could be:

PŘED-pla-tse-naa KAR-ta NE-nee tak DRA-haa YA-ko MO-bil-nee TA-rif

That is only approximate, but it gives the stress and vowel length fairly well.