Pies czeka przy drzwiach, bo widzi smycz i wie, że zaraz pójdziemy na spacer.

Because pies is the subject of the sentence, so it is in the nominative case.

• pies = dog, nominative singular
• psa = dog, accusative/genitive singular

Here the dog is the one doing the actions: czeka, widzi, wie. So pies is the correct form.

Czeka is the 3rd person singular present form of czekać, meaning to wait.

It matches pies because pies is singular:

• ja czekam = I wait
• ty czekasz = you wait
• on/ona/ono czeka = he/she/it waits

So Pies czeka means The dog is waiting or The dog waits, depending on context.

Because the preposition przy requires the locative case.

• przy = by, next to, at
• drzwiach = locative plural of drzwi

So:

• drzwi = door / doors
• przy drzwiach = by the door

This is a very common Polish pattern: certain prepositions always force a certain case, and przy takes the locative.

Because drzwi is a plural-only noun in Polish. This is called pluralia tantum.

Even when English uses singular door, Polish usually uses drzwi, which is grammatically plural:

• te drzwi = this door / these doors
• przy drzwiach = by the door

So learners often have to remember that drzwi behaves like a plural noun even when the meaning is singular in English.

Because widzieć takes a direct object in the accusative, and the noun smycz happens to have the same form in the nominative and accusative singular.

So:

• smycz = leash, nominative singular
• widzi smycz = sees a/the leash, accusative singular

This happens with some feminine nouns ending in a consonant, such as mysz, noc, twarz, smycz.

So the case does change grammatically, but the visible form stays the same.

Bo means because.

Yes, ponieważ could also work, but there is a style difference:

• bo = very common, natural, everyday
• ponieważ = a bit more formal or written

So this sentence sounds very natural with bo: Pies czeka przy drzwiach, bo widzi smycz...

Because in Polish, subordinate clauses introduced by bo and że are normally separated by commas.

So the sentence is divided like this:

• Pies czeka przy drzwiach,
• bo widzi smycz
• i wie, że zaraz pójdziemy na spacer.

This is standard Polish punctuation.

Że means that and introduces a clause after verbs like know, think, say, see, and so on.

So:

• wie = knows
• wie, że... = knows that...

In English, that is often omitted:

• He knows we’ll go for a walk soon

In Polish, że is normally expressed here:

• wie, że zaraz pójdziemy na spacer

It looks present in form, but because pójść is a perfective verb, its present forms actually refer to the future.

So:

• pójść = to go, perfective
• pójdziemy = we will go

This is a very important Polish pattern: perfective verbs do not normally have a true present meaning. Their present-looking forms are used for future actions.

Both can be possible in some contexts, but they are not exactly the same.

• idziemy from iść can mean we are going or we’re going soon
• pójdziemy from pójść means we will go, often with a sense of starting or completing that one trip

In this sentence, pójdziemy na spacer sounds very natural because the dog knows that the walk will begin in a moment.

Because Polish often drops subject pronouns when the verb ending already makes the subject clear.

Pójdziemy already tells you the subject is we.

So:

• pójdziemy = we will go
• my pójdziemy = we will go, with extra emphasis on we

Polish usually leaves out my unless it is needed for contrast or emphasis.

Zaraz usually means in a moment, very soon, or shortly.

In this sentence:

• że zaraz pójdziemy na spacer = that we’ll go for a walk very soon

It does not always mean literally right now this second. It often means something is about to happen.

Because iść/pójść na spacer is the normal Polish expression for to go for a walk.

Here na is used with the accusative after a verb of motion:

• iść na spacer
• pójść na spacer

Also, spacer is masculine inanimate, and in the singular its accusative form is the same as the nominative, so you see spacer, not a changed ending.

Not completely. Polish word order is more flexible than English, because case endings show grammatical relationships.

The given order is natural and neutral: Pies czeka przy drzwiach, bo widzi smycz i wie, że zaraz pójdziemy na spacer.

But other orders are possible for emphasis, for example: Przy drzwiach czeka pies...

That version puts more focus on the location. The original sentence sounds smooth and normal for everyday narration.