On poważnie myśli o zmianie pracy, a ja szczerze mówiąc rozumiem jego decyzję.

That’s true: Polish often omits subject pronouns because the verb ending already shows who the subject is.

• myśli = he/she/it is thinking
• rozumiem = I understand

So the sentence could also be:

• Poważnie myśli o zmianie pracy, a ja szczerze mówiąc rozumiem jego decyzję.

Including on can do a few things:

• make the subject extra clear
• add slight emphasis or contrast
• help the sentence sound more balanced at the start

So on is not required, but it is perfectly natural.

Poważnie myśli o... means he is seriously thinking about... or he is seriously considering...

Breakdown:

• poważnie = seriously
• myśli = thinks / is thinking
• o = about

So this is not just a random thought. It suggests real consideration.

A close English equivalent is:

• He is seriously considering changing jobs.

Because the preposition o in the meaning about takes the locative case.

So:

• o zmianie = about a change
• not o zmianę

This is an important pattern:

• myśleć o kimś / o czymś = to think about someone / something

Examples:

• Myślę o wakacjach. = I’m thinking about vacation.
• Myślimy o nowym mieszkaniu. = We’re thinking about a new apartment.

Here:

• zmiana = change
• locative singular = zmianie

Because zmiana pracy means a change of job/work, and after the noun zmiana, the thing being changed is commonly put in the genitive.

So:

• zmiana = change
• praca = work / job
• genitive singular of praca = pracy

That gives:

• zmiana pracy = change of job / changing jobs

Then after o, zmiana changes to locative:

• o zmianie pracy = about changing jobs / about a job change

So the two case layers are:

1. zmiana pracy
2. o zmianie pracy

Yes. In natural English, zmiana pracy is usually best understood as:

• changing jobs
• a job change
• sometimes changing work

In Polish, praca can mean both work and job, depending on context. In this sentence, job is the more natural English choice.

So:

• On poważnie myśli o zmianie pracy
  = He is seriously thinking about changing jobs.

A is a very common Polish conjunction that often links two clauses with a slight sense of contrast, comparison, or topic shift.

In this sentence:

• On ... , a ja ...

This feels like:

• He ..., and I ...
• or He ..., while I ...
• or He ..., and as for me ...

It is not exactly the same as i.

• i = simple and
• a = and / while / whereas / as for, often with contrast

Here a works very naturally because the sentence compares his situation with my reaction.

Literally, szczerze mówiąc means speaking honestly or honestly speaking.

In natural English, we usually translate it as:

• honestly
• to be honest
• frankly speaking

It is a fixed expression used as a parenthetical comment, meaning the speaker is adding a personal, candid remark.

Examples:

• Szczerze mówiąc, nie wiem. = Honestly, I don’t know.
• Ja, szczerze mówiąc, się zgadzam. = I honestly agree.

In your sentence, it softens and personalizes the statement:

• a ja szczerze mówiąc rozumiem jego decyzję
  = and honestly, I understand his decision

Because szczerze mówiąc is treated as a parenthetical expression—something inserted into the sentence rather than a core grammatical part of it.

So Polish normally separates it with commas:

• a ja, szczerze mówiąc, rozumiem jego decyzję
• or sometimes, less fully punctuated in informal writing: a ja szczerze mówiąc rozumiem jego decyzję

The fully punctuated version is often considered clearer and more standard because szczerze mówiąc interrupts the main clause.

Main clause:

• ja rozumiem jego decyzję

Inserted comment:

• szczerze mówiąc

Because rozumieć takes a direct object, and direct objects in Polish are usually in the accusative case.

So:

• dictionary form: decyzja = decision
• accusative singular: decyzję

That gives:

• rozumiem decyzję = I understand the decision
• rozumiem jego decyzję = I understand his decision

This is a basic verb pattern:

• rozumieć kogo? co? = to understand whom? what?

Because jego means his, while swój / swoją / swoje is a reflexive possessive that refers back to the subject of the same clause.

In the clause:

• ja ... rozumiem jego decyzję

the subject is ja = I.

So:

• swoją decyzję would mean my own decision
• jego decyzję means his decision

That is exactly what we want here: I understand his decision.

Compare:

• On rozumie swoją decyzję. = He understands his own decision.
• Ja rozumiem jego decyzję. = I understand his decision.

This is a very important difference in Polish.

Myśleć is imperfective, and that fits because this is an ongoing mental process: he is in the process of considering something.

• myśli o zmianie pracy = he is thinking about changing jobs

This is not a completed act. It is open-ended and ongoing.

A different verb could change the meaning slightly:

• rozważa zmianę pracy = he is considering a job change
• planuje zmianę pracy = he is planning a job change
• zamierza zmienić pracę = he intends to change jobs

Your original sentence suggests he has not necessarily decided yet, but he is thinking seriously about it.

Yes. Polish word order is relatively flexible, though each version may sound slightly different in emphasis.

Original:

• On poważnie myśli o zmianie pracy, a ja szczerze mówiąc rozumiem jego decyzję.

Other possible versions:

• Poważnie myśli o zmianie pracy, a ja, szczerze mówiąc, rozumiem jego decyzję.
• On o zmianie pracy myśli poważnie, a ja rozumiem jego decyzję.
• Ja, szczerze mówiąc, rozumiem jego decyzję.

The most neutral and natural version is usually closest to the original. Reordering is possible, but some versions may sound more literary, more emphatic, or less natural in everyday speech.

Yes, it sounds natural and idiomatic.

A very polished version would probably be written as:

• On poważnie myśli o zmianie pracy, a ja, szczerze mówiąc, rozumiem jego decyzję.

That version just adds commas around szczerze mówiąc, which many teachers and editors would prefer.

But the wording itself is completely natural:

• poważnie myśli o zmianie pracy is a normal way to say is seriously considering changing jobs
• szczerze mówiąc is a common conversational phrase
• rozumiem jego decyzję is standard and clear

Usually, myśleć o zmianie pracy is the more natural choice here.

Both myśleć o and myśleć nad exist, but they are not always interchangeable.

• myśleć o czymś = think about something
• myśleć nad czymś = think over something, reflect on something, work mentally on a problem

For a personal life decision like this, myśli o zmianie pracy sounds more idiomatic.

Examples:

• Myślę o wyjeździe. = I’m thinking about leaving.
• Myślę nad rozwiązaniem problemu. = I’m thinking over a solution to the problem.

So here, o zmianie pracy is the best fit.