Porovnal jsem dva mobilní tarify a koupil předplacenou kartu.

In Czech, the subject pronoun is often omitted when it is clear from the verb form.
Here, jsem already tells you the speaker is I.

So:

• Porovnal jsem = I compared
• Já jsem porovnal = also possible, but já adds emphasis, like I compared rather than someone else.

The version without já is the most neutral and natural here.

This is a very common Czech pattern in the past tense.

The past tense is made with:

• a past participle: porovnal
• plus an auxiliary form of být: jsem

In normal Czech word order, the auxiliary jsem often comes in the second position of the clause. So:

• Porovnal jsem dva mobilní tarify.

sounds natural.

You may also hear other word orders in real speech, but this one is standard and very common.

Because both verbs have the same subject and are linked by a (and), Czech can use the auxiliary only once.

So:

• Porovnal jsem dva mobilní tarify a koupil předplacenou kartu.

means:

• I compared two mobile plans and bought a prepaid card.

The jsem applies to both verbs. This is very normal in Czech.

You could repeat it, but it would usually sound unnecessary:

• Porovnal jsem dva mobilní tarify a koupil jsem předplacenou kartu.

This is still grammatical, just less compact.

Both phrases are direct objects, so they are in the accusative case.

Here is what happens:

• dva mobilní tarify = accusative plural
• předplacenou kartu = accusative singular

You can see the case especially clearly in kartu, because the basic dictionary form is:

• karta = nominative singular
• kartu = accusative singular

And with the adjective:

• předplacená karta = nominative
• předplacenou kartu = accusative

So the endings change because the speaker bought the card.

Because dva is used with masculine and neuter nouns, while dvě is used with feminine nouns.

Here:

• tarif is masculine inanimate
• plural accusative: dva tarify

Compare:

• dva tarify = two plans
• dvě karty = two cards

So the choice depends on the noun’s grammatical gender.

Because karta is the nominative form, but after the verb koupit (to buy), the noun is a direct object, so Czech uses the accusative.

For this feminine noun:

• karta = nominative singular
• kartu = accusative singular

So:

• Karta je nová. = The card is new.
• Koupil jsem kartu. = I bought a card.

Because the adjective must agree with the noun it describes.

The noun is:

• karta = feminine singular

And in this sentence it is in the accusative singular:

• kartu

So the adjective must also be feminine singular accusative:

• předplacená karta = nominative
• předplacenou kartu = accusative

This agreement of gender + number + case is one of the basic rules of Czech adjectives.

Because not all adjective forms look different in every case.

In dva mobilní tarify, the adjective mobilní happens to have the same visible form here.
In předplacenou kartu, the adjective shows a more obvious accusative ending.

So the difference is not that one adjective declines and the other does not.
Both decline, but their forms look different depending on the adjective type and ending.

In this sentence:

• mobilní agrees with tarify
• předplacenou agrees with kartu

Yes. They are understood as completed actions.

• porovnal = compared / finished comparing
• koupil = bought / completed the purchase

That fits the sentence well, because it describes two finished actions in sequence.

The imperfective partners would be:

• porovnávat = to be comparing / to compare repeatedly
• kupovat = to be buying / to buy repeatedly

If you wanted to stress process or repetition, you would use the imperfective forms instead.

Yes. Czech word order is more flexible than English, because case endings show grammatical relationships.

The given sentence is a neutral, natural version:

• Porovnal jsem dva mobilní tarify a koupil předplacenou kartu.

But other orders are possible for emphasis, for example:

• Dva mobilní tarify jsem porovnal a koupil předplacenou kartu.
• Předplacenou kartu jsem koupil po porovnání dvou mobilních tarifů.

However, not every possible order sounds equally natural in every context. The original sentence is a good standard model.

A means and. It joins two coordinated verbs with the same subject:

• porovnal jsem
• koupil

So the structure is:

• I compared X and bought Y.

It is a simple coordinating conjunction, just like English and.

Because tarif is a masculine inanimate noun, and its plural nominative/accusative form is tarify.

So:

• tarif = plan / tariff
• tarify = plans / tariffs

Since dva is present, the phrase clearly means two plans.

Yes. The forms porovnal and koupil are the masculine singular past participles.

So this sentence, as written, is spoken by a male speaker.

A female speaker would say:

• Porovnala jsem dva mobilní tarify a koupila předplacenou kartu.

That is an important feature of Czech past tense: it agrees with the speaker’s gender in the singular.