Po południu idę do sekretariatu, żeby zostawić teczkę dla sekretarki.

Because po here is a preposition that requires the locative case, and południe changes to południu in the locative.

So:

• południe = noon / midday
• po południu = in the afternoon / after noon

This is a very common fixed expression in Polish. A learner should remember po południu as a whole phrase.

Idę is used for a specific trip or movement in one direction, including something you are about to do soon.

So Po południu idę do sekretariatu means something like This afternoon I’m going to the office.

By contrast, chodzę usually means:

• I go regularly
• I walk around
• I am in the habit of going

So:

• Idę do sekretariatu = I’m going to the office
• Chodzę do sekretariatu = I go to the office regularly / I keep going there

This is a very common Polish distinction:

• iść / jechać = one trip, one direction
• chodzić / jeździć = habitual or repeated movement

Because do is the normal preposition used with verbs of motion when you mean to a place, and it requires the genitive case.

The noun is:

• sekretariat = office / administration office / secretary’s office

After do, it becomes:

• do sekretariatu

So:

• Idę do sekretariatu = I’m going to the office

Compare:

• w sekretariacie = in the office

So the case changes depending on whether you mean movement to a place or location in a place.

They are different words:

• sekretariat = the office, front office, administrative office
• sekretarka = a female secretary / receptionist / administrative assistant

So in this sentence:

• idę do sekretariatu = I’m going to the office
• dla sekretarki = for the secretary

English learners sometimes confuse them because both are related to secretary, but Polish keeps the place and the person clearly separate.

Because żeby introduces a subordinate clause, and in Polish such clauses are normally separated by a comma.

So:

• Idę do sekretariatu, żeby zostawić teczkę...

This is standard Polish punctuation.

You will often see the same with words like:

• że
• bo
• kiedy
• jeśli
• żeby

For English speakers, this can feel slightly more regular than English punctuation: in Polish, the comma before a subordinate clause is usually required.

Żeby means so that, in order to, or so as to, depending on the sentence.

Here it introduces a purpose:

• żeby zostawić teczkę = in order to leave the folder / to drop off the folder

So the structure is:

• [main action] + żeby + [purpose]

In this sentence:

• Idę do sekretariatu = I’m going to the office
• żeby zostawić teczkę = in order to leave the folder there

This is one of the most common ways to express purpose in everyday Polish.

Because zostawić is the perfective form, and it fits a single completed action.

Here the speaker plans one concrete action:

• go to the office
• leave/drop off the folder
• action completed

So zostawić is natural.

Compare:

• zostawić = to leave, once, successfully, to completion
• zostawiać = to be leaving regularly, repeatedly, or in a more ongoing/imperfective sense

Examples:

• Chcę zostawić teczkę. = I want to leave/drop off the folder.
• Często zostawiam teczkę w sekretariacie. = I often leave the folder in the office.

English does not mark aspect this way, so this is a very common question for learners.

Because teczkę is the accusative singular form, and it is the direct object of zostawić.

The basic form is:

• teczka = folder / file / briefcase-like folder

But after a verb like zostawić when it is the thing being left, Polish uses the accusative:

• zostawić teczkę

Since teczka is a feminine noun ending in -a, the accusative singular usually changes to -ę:

• teczka → teczkę

This is a very common feminine noun pattern.

Because dla means for, and it takes the genitive case.

So:

• sekretarka = secretary
• dla sekretarki = for the secretary

This phrase tells us who the folder is intended for.

A useful contrast:

• teczkę dla sekretarki = a folder for the secretary
• zostawić sekretarce teczkę = leave the secretary the folder / leave the folder with the secretary

Both are possible, but they emphasize slightly different things.
In your sentence, dla sekretarki focuses on the folder being meant for her.

Yes. Polish word order is much more flexible than English word order, because grammatical endings show the roles of the words.

This sentence could be rearranged in several ways, for example:

• Idę po południu do sekretariatu, żeby zostawić teczkę dla sekretarki.
• Do sekretariatu idę po południu, żeby zostawić teczkę dla sekretarki.

The original version:

• Po południu idę do sekretariatu, żeby zostawić teczkę dla sekretarki.

sounds natural and puts the time expression first, which often sets the scene nicely.

So yes, the order can change, but different orders may shift the emphasis.

Yes, absolutely. It is very natural.

When the subject of both actions is the same, Polish often uses żeby + infinitive:

• Idę do sekretariatu, żeby zostawić teczkę.
• Przyszedłem, żeby pomóc.
• Otworzyła okno, żeby przewietrzyć pokój.

This is similar to English to... or in order to...

Polish can also use a finite verb after żeby, especially when the subject changes:

• Daję ci teczkę, żebyś zaniósł ją do sekretariatu. = I’m giving you the folder so that you take it to the office.

So in your sentence, żeby zostawić is normal because the same person is both going and leaving the folder.