Na długi spacer zakładam psu szelki, a w autobusie musi mieć także kaganiec.

Because na can take different cases depending on the meaning.

• na + accusative often means for, onto, or for the purpose of
• na + locative often means on / at / during a place or event

Here, na długi spacer means for a long walk, so spacer is in the accusative:

• długi spacer → accusative: długi spacer

Compare:

• Idę na długi spacer. = I’m going for a long walk.
• Rozmawiamy na długim spacerze. = We’re talking during/on a long walk.

So in this sentence, it is about the occasion or purpose: for a long walk.

Psu is the dative singular of pies (dog).

It appears because Polish often uses this pattern:

• zakładać komuś coś = to put something on someone

So:

• zakładam psu szelki literally means I put the harness on the dog
• structure: I put [something] on [someone]
• in Polish: [something]
  • on whom? → that person/animal is often in the dative

Forms of pies:

• nominative: pies
• dative: psu

So psu means to the dog / on the dog in this context.

Because szelki is normally used only in the plural in Polish.

This is called a plural-only noun (pluralia tantum). Even though in English harness is singular, Polish uses the plural form:

• szelki = harness / braces / suspenders, depending on context

So:

• zakładam psu szelki = I put a harness on the dog

You should learn szelki as a plural noun, much like:

• drzwi = door
• nożyczki = scissors

That is why the sentence does not use a singular form like szelka for this meaning.

It is accusative, but for this noun the accusative singular looks the same as the nominative singular.

• nominative: kaganiec
• accusative: kaganiec

This happens because kaganiec is a masculine inanimate noun, and in Polish:

• masculine inanimate nouns usually have
  • nominative singular = accusative singular

Since mieć takes the accusative, the sentence is grammatically:

• musi mieć kaganiec

Even though the form does not visibly change, the case is still accusative.

Because Polish has no articles like English a/an/the.

So kaganiec can mean:

• a muzzle
• the muzzle

Which one is meant depends on the context.

In this sentence, English would naturally say:

• must also have a muzzle

But Polish simply says:

• musi mieć także kaganiec

This is completely normal.

Because w means in and here it refers to being in/on the bus, so Polish uses the locative case.

• autobus = bus
• locative singular: autobusie

So:

• w autobusie = in/on the bus

This is a standard pattern:

• w domu = in the house
• w sklepie = in the shop
• w samochodzie = in the car

Even though English often says on the bus, Polish uses w autobusie.

Because zakładam is imperfective, and it fits a habitual or general action.

• zakładać = imperfective
• założyć = perfective

In this sentence, the speaker is describing what they normally do in a certain situation:

• For a long walk, I put a harness on the dog...

That sounds like a repeated or usual action, so zakładam is natural.

Compare:

• Zakładam psu szelki. = I put / I am putting a harness on the dog. / I usually put a harness on the dog.
• Założę psu szelki. = I will put a harness on the dog / I’ll put the harness on the dog.

So zakładam fits the general statement better.

Because Polish often omits subjects when they are clear from the context.

Here, the understood subject is the dog.

The first part mentions the dog explicitly:

• zakładam psu szelki

Then the second part continues talking about that same dog:

• a w autobusie musi mieć także kaganiec

So the full idea is:

• the dog must also have a muzzle on the bus

Polish does not need to repeat pies if it is already obvious.

A more explicit version could be:

• a w autobusie pies musi mieć także kaganiec

But that is less natural unless you want extra emphasis.

It refers to the dog.

Even though Polish leaves the subject unstated, the context makes it clear. The sentence is about what the dog wears:

• first, the dog gets a harness
• then, on the bus, the dog must also have a muzzle

So:

• musi mieć także kaganiec = it/the dog must also have a muzzle

It does not mean the speaker must have one.

Yes, także and też can both mean also / too.

In this sentence:

• musi mieć także kaganiec = must also have a muzzle

You could also say:

• musi też mieć kaganiec

The difference is mostly about style and emphasis:

• także is a bit more formal or careful
• też is very common in everyday speech

Both are correct.

Both are possible.

Polish word order is more flexible than English, and moving elements changes the focus or emphasis, not the basic meaning.

• Na długi spacer zakładam psu szelki
  This starts with the situation/context: for a long walk
• Zakładam psu szelki na długi spacer
  This starts with the action: I put a harness on the dog

The version in your sentence sounds natural because it first sets the context, then says what happens in that context.

Yes. Polish allows some flexibility here too.

You can say:

• zakładam psu szelki
• zakładam szelki psu

Both are understandable, but zakładam psu szelki often sounds a bit more natural in neutral speech.

The first version follows a common pattern:

• verb + recipient/wearer + item

So:

• zakładam dziecku czapkę = I put a hat on the child
• zakładam psu szelki = I put a harness on the dog

Not exactly. Here it means to put on something onto someone/something.

This is an important distinction:

• nosić = to wear, to have on regularly
• mieć na sobie = to be wearing, to have on
• zakładać = to put on

So:

• Zakładam psu szelki. = I put the harness on the dog.
• Pies nosi szelki. = The dog wears a harness.
• Pies ma na sobie szelki. = The dog has a harness on.

In your sentence, the action is the act of putting them on, so zakładam is the right verb.

Because a in Polish often links two related ideas with a slight contrast or shift of context.

Here the sentence means something like:

• For a long walk I put a harness on the dog, and in the bus it must also have a muzzle.

The conjunction a works well because the sentence moves from one situation to another:

• walking
• being on the bus

It is not a strong contrast like but, but it does mark a transition.

Using i would sound more like simply adding one fact to another. A is more natural here.

Yes, but długi adds the meaning long.

• na spacer = for a walk
• na długi spacer = for a long walk

The adjective długi must agree with spacer:

• spacer is masculine singular
• accusative singular masculine inanimate = same as nominative here
• so: długi spacer

Yes, absolutely.

You could say:

• Na długi spacer zakładam psu szelki, a w autobusie pies musi mieć także kaganiec.

That is grammatically correct. But Polish often avoids repeating nouns when they are obvious from context. Since psu already clearly introduces the dog, leaving out pies in the second clause sounds more natural and less repetitive.

It is in the accusative singular.

The phrase is:

• na długi spacer

Here na expresses purpose/direction, so it takes the accusative.

For spacer:

• nominative singular: spacer
• accusative singular: spacer

The form does not change because spacer is masculine inanimate, and those nouns often have the same nominative and accusative form in the singular.

So although it looks unchanged, it is accusative.

Yes, very much so.

Because of:

• zakładam (imperfective, habitual/general)
• the overall structure describing what happens in certain situations

the sentence naturally sounds like:

• When going for a long walk, I put a harness on the dog, and on the bus it must also have a muzzle.

So it reads more like a routine or rule than a one-time event.