Ten czasownik jest prosty, ale ten rzeczownik ma trudniejszą odmianę.

No. Polish does not have articles like a or the.

Ten is a demonstrative word, usually meaning this or sometimes that, depending on context. So:

• ten czasownik = this verb
• ten rzeczownik = this noun

A learner might expect every noun to need something like the, but Polish does not work that way.

Because both nouns are:

• masculine
• singular
• nominative

The demonstrative must agree with the noun in gender, number, and case.

Here the basic nominative forms are:

• ten for masculine
• ta for feminine
• to for neuter

So:

• ten czasownik
• ten rzeczownik

If the noun were feminine, you would use ta. For example: ta odmiana.

Yes, very often.

You could say:

Czasownik jest prosty, ale rzeczownik ma trudniejszą odmianę.

That sounds more general: A verb is simple, but a noun has a more difficult declension/inflection or The verb is simple, but the noun has a more difficult declension, depending on context.

Using ten makes it feel more specific or contrastive, as if you are pointing to a particular verb and a particular noun.

They are not all in the same case.

• czasownik — nominative, because it is the subject of jest
• rzeczownik — nominative, because it is the subject of ma
• odmianę — accusative, because it is the direct object of ma

So the structure is:

• Ten czasownik jest prosty
• ten rzeczownik ma trudniejszą odmianę

A very common pattern in Polish is:

• subject in nominative
• direct object in accusative

Because prosty agrees with czasownik, which is:

• masculine
• singular
• nominative

So the adjective also has to be masculine singular nominative:

• prosty czasownik
• ten czasownik jest prosty

After jest with an adjective, Polish normally uses the nominative form of the adjective.

Because the adjective trudniejszą must agree with odmianę.

The noun odmiana is:

• feminine
• singular
• here in the accusative

So the adjective must also be:

• feminine
• singular
• accusative

That gives:

• trudniejszą odmianę

Compare:

• trudniejsza odmiana = nominative
• ma trudniejszą odmianę = accusative

This agreement is one of the most important things to watch in Polish.

It comes from the adjective trudny = difficult.

Step by step:

• trudny = difficult
• trudniejszy = more difficult

Then that comparative adjective is changed to match odmianę:

• feminine singular nominative: trudniejsza
• feminine singular accusative: trudniejszą

So in this sentence:

• ma trudniejszą odmianę = has a more difficult declension/inflection

Because the sentence is being more specific.

• rzeczownik jest trudniejszy = the noun is more difficult
• rzeczownik ma trudniejszą odmianę = the noun has a more difficult inflection/declension pattern

So the difficulty is not necessarily about the whole noun in every sense. It is specifically about its odmiana.

This is a natural Polish way to say that one word is harder because its forms change in a more complicated way.

Because mieć = to have, and the thing that is had is a direct object. Direct objects are very often in the accusative in Polish.

So:

• ma odmianę = has a declension/inflection
• ma trudniejszą odmianę = has a more difficult declension/inflection

This is a very common pattern:

• mam książkę
• ma samochód
• rzeczownik ma trudniejszą odmianę

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

The sentence as given:

Ten czasownik jest prosty, ale ten rzeczownik ma trudniejszą odmianę.

is a neutral, clear order.

You can change word order for emphasis, but it may sound more marked. For example, you could move things around to emphasize trudniejszą odmianę or the contrast with ale.

For learners, the given order is a very good model because it is straightforward and natural.

Ale means but and connects the two clauses in contrast:

• Ten czasownik jest prosty
• ale
• ten rzeczownik ma trudniejszą odmianę

So the speaker is contrasting:

• the verb being simple with
• the noun having a more difficult inflection pattern

It is a very common coordinating conjunction in Polish.