Minun pitää mennä postiin ennen kuin se sulkeutuu.

Because pitää in this meaning is used in a structure where the person who has the obligation appears in the genitive:

• minun pitää = I have to / I must
• sinun pitää = you have to
• hänen pitää = he/she has to

So minun is not the subject in the usual nominative form minä. This is a very common Finnish pattern with pitää when it means must / have to.

Compare:

• Minä menen. = I go / I am going.
• Minun pitää mennä. = I have to go.

In the second sentence, mennä is the main action, and pitää expresses necessity.

Here pitää means to have to, must, or need to.

That is different from another common meaning of pitää:

• pitää jostakin = to like something
  
  • Pidän kahvista. = I like coffee.

But in minun pitää mennä, it does not mean like. It means obligation or necessity:

• Minun pitää mennä. = I have to go.

So Finnish learners need to recognize that pitää has more than one meaning depending on the structure.

Because after pitää in this construction, the main action is expressed with the first infinitive:

• Minun pitää mennä. = I have to go.
• Sinun pitää syödä. = You have to eat.
• Meidän pitää lähteä. = We have to leave.

So mennä stays in the infinitive form, not a conjugated form like menen.

You can think of it as similar to English have to go, where go also stays in a basic form after have to.

Postiin is the illative form, which often means into or to a place.

• posti = post office / mail / post
• postissa = in the post office
• postiin = into the post office / to the post office

Since the sentence talks about going to the post office, Finnish uses the form that shows movement into the place:

• mennä postiin = to go to the post office

This is very common with places you enter:

• mennä kouluun = go to school
• mennä kauppaan = go to the store
• mennä pankkiin = go to the bank

That is how the illative case is formed for many words of this type.

For posti, the illative is:

• posti → postiin

This -iin ending often appears with words whose basic form ends in -i.

Some similar examples:

• pankki → pankkiin
• hotelli → hotelliin
• bussi → bussiin

So postiin is just the correct case form meaning to the post office.

Ennen kuin means before when it introduces a clause.

In this sentence:

• ennen kuin se sulkeutuu = before it closes

This is different from using just ennen with a noun:

• ennen iltaa = before evening
• ennen lähtöä = before departure

But when there is a full clause with its own verb, Finnish normally uses ennen kuin:

• ennen kuin menen = before I go
• ennen kuin hän tulee = before he/she comes
• ennen kuin se sulkeutuu = before it closes

So kuin is needed here because what follows is a whole clause, not just a noun phrase.

Yes, here se means it, referring to the post office.

Finnish uses se very commonly for things, and in everyday spoken Finnish it is also often used for people. In standard written Finnish, though:

• se = it / that
• hän = he / she

In this sentence, se is perfectly natural because the thing closing is the post office.

Because sulkeutua means to close in the sense of to become closed, while sulkea usually means to close something.

Compare:

• Posti sulkeutuu kello viisi. = The post office closes at five.
• Työntekijä sulkee oven. = The employee closes the door.

So:

• sulkea = to close something
• sulkeutua = to close / to get closed

In your sentence, the post office is not actively closing something else; it itself becomes closed. That is why sulkeutuu is used.

Finnish usually does not have a separate future tense the way English does. Present tense often covers present and future meaning.

So:

• Minun pitää mennä postiin ennen kuin se sulkeutuu.

uses present-tense forms, even though the meaning is future-oriented.

This is very normal in Finnish. Context tells you the time.

Other examples:

• Menen huomenna. = I will go tomorrow.
• Juna lähtee pian. = The train will leave soon.
• Soitan sinulle illalla. = I will call you in the evening.

Not exactly.

• mennä postiin usually means going into the post office
• mennä postille usually means going to the post office area / to the post office, often with an outside or destination sense

In many real-life situations, people may use both in ways that overlap, but postiin strongly suggests entering the building, which fits this sentence well because the point is getting there before it closes.

This same contrast appears with many locations:

• mennä kouluun = go into school / go to school
• mennä koululle = go to the school area / school building

Finnish word order is often more flexible than English, because endings show grammatical relationships.

This sentence follows a very natural Finnish order:

• Minun pitää mennä postiin ennen kuin se sulkeutuu.

Literally, the parts are roughly:

• minun pitää = I have to
• mennä postiin = go to the post office
• ennen kuin se sulkeutuu = before it closes

That order is standard and natural in Finnish. English learners sometimes expect a more fixed pattern, but Finnish relies a lot on case endings and verb forms instead of strict word order.

Yes. That is another way to express necessity.

• Minun pitää mennä postiin.
• Minun on mentävä postiin.

Both mean roughly I have to go to the post office.

However, there is a style difference:

• minun pitää mennä is very common and neutral
• minun on mentävä sounds a bit more formal, emphatic, or literary

So for everyday speech and basic conversation, minun pitää mennä is usually the more common choice.