Ona długo nie mogła się zdecydować, który fotel postawić przy oknie, a który przy lampie.

Yes. Polish often drops subject pronouns when the verb already shows who the subject is.

• nie mogła already tells you the subject is she because mogła is feminine singular past tense.
• So Długo nie mogła się zdecydować... is completely natural.

Including ona adds emphasis or contrast, for example:

• Ona długo nie mogła się zdecydować, ale on od razu wiedział.

So ona is not required here, but it is possible.

Because the subject is ona (she), so the past tense must be feminine singular.

Past tense forms of móc in singular are:

• mógł = he could
• mogła = she could
• mogło = it could

So:

• Ona nie mogła = She couldn’t
• On nie mógł = He couldn’t

Długo means for a long time or long in this context.

So:

• Ona długo nie mogła się zdecydować = She couldn’t decide for a long time.

Its position is fairly flexible. For example:

• Ona długo nie mogła się zdecydować
• Długo nie mogła się zdecydować
• Nie mogła się długo zdecydować

All are possible, though they may differ slightly in emphasis.

The verb is zdecydować się na coś or simply zdecydować się, meaning to decide or more literally to make up one’s mind.

In this sentence:

• nie mogła się zdecydować = she couldn’t decide / she couldn’t make up her mind

The się is part of the verb expression. You should learn it together:

• zdecydować się = to decide
• decydować się = to be deciding / to decide (imperfective)

So this is not just zdecydować by itself in the same sense; the reflexive form is the normal one here.

Yes, both are possible:

• mogła się zdecydować
• mogła zdecydować się

But się often appears early in the clause, and mogła się zdecydować sounds very natural.

Polish word order is flexible, but short unstressed words like się often avoid standing at the very beginning of a clause and frequently come near the verb.

So the sentence as written is standard and natural.

This is a very common learner question.

• jaki fotel? = what kind of armchair? / what sort of armchair?
• który fotel? = which armchair? (from a known set of options)

In this sentence she is choosing between specific armchairs and deciding where to place each one, so który is correct:

• który fotel postawić przy oknie, a który przy lampie
• which armchair to put by the window, and which by the lamp

Because postawić is perfective and refers to a completed action: to place / to put down.

Here she is deciding about a specific result:

• which armchair to place by the window
• which one to place by the lamp

That makes the perfective verb natural.

Compare:

• postawić = to place, to put down once, with completion
• stawiać = to be placing, to place repeatedly, habitually, or without focusing on completion

So który fotel postawić... means which armchair to place..., not a repeated or ongoing action.

Yes. Fotel usually means armchair or easy chair, not an ordinary chair.

Compare:

• krzesło = chair
• fotel = armchair

So the sentence is specifically about armchairs, not regular chairs.

The preposition przy takes the locative case when it means by, next to, or near.

So:

• okno → przy oknie
• lampa → przy lampie

That is why the endings change.

Examples:

• przy stole = by the table
• przy drzwiach = by the door
• przy oknie = by the window

Because they are in the locative singular after przy.

Here are the base forms and the locative forms:

• okno → oknie
• lampa → lampie

This is a normal case change in Polish noun declension.

So:

• przy oknie = by the window
• przy lampie = by the lamp

You do not need to translate the case ending directly into English; it is just how Polish marks the noun after this preposition.

Because Polish often leaves out words that are easy to understand from context.

The full version would be:

• który fotel postawić przy oknie, a który fotel postawić przy lampie

But that sounds repetitive, so Polish naturally shortens it to:

• który fotel postawić przy oknie, a który przy lampie

The missing parts are understood:

• a który [fotel postawić] przy lampie

English does the same:

• which armchair to put by the window, and which by the lamp

Here a connects two contrasted parts:

• which armchair by the window
• and which by the lamp

It is often translated as and, but it has a slight contrastive feel, something like:

• and which one by the lamp
• while which one by the lamp
• and which one, on the other hand, by the lamp

In many contexts, a links two parallel or contrasted ideas more naturally than i.

It is functioning with fotel, and here the phrase is effectively the object of postawić:

• który fotel postawić = which armchair to place

Normally that would be accusative. But fotel is masculine inanimate, and for masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: który fotel
• accusative: który fotel

They look identical here, even though the function is object-like.

Yes, in some contexts it could, but the nuance changes a bit.

• postawić = to put, set down, place
• ustawić = to position, arrange, set in place

If the focus is simply putting the armchairs somewhere, postawić works well. If the focus is arranging them in a certain layout, ustawić might sound more natural.

So both can be possible, but postawić is perfectly normal in this sentence.

Yes, exactly. The structure is:

• nie mogła się zdecydować = she couldn’t decide
• który fotel postawić przy oknie = which armchair to put by the window
• a który przy lampie = and which one by the lamp

So the decision is about assigning one armchair to one location and the other to the other location.