Jeśli dostaniemy premię, kupię większy regał do salonu i nowe firanki do sypialni.

Polish usually drops subject pronouns when they are not needed, because the verb ending already shows the person and number.

• dostaniemy = we will get
• kupię = I will buy

So adding my or ja would usually only be for emphasis:

• Jeśli my dostaniemy premię, ja kupię...
  This sounds contrastive, as if you are stressing we and I.

Because the two clauses have different subjects:

• Jeśli dostaniemy premię = If we get a bonus
• kupię większy regał... = I will buy a bigger bookcase...

English makes this obvious with we and I. In Polish, the verb endings do that job:

• -my in dostaniemy = we
• -ę in kupię = I

This is a very common Polish pattern. The verb dostać is perfective, and perfective verbs do not normally have a present tense with present meaning. Their present-looking forms are used for the future.

So:

• dostajemy = we are getting / we get (imperfective, present)
• dostaniemy = we will get (perfective, future)

That is why Jeśli dostaniemy premię means If we get / if we receive a bonus in the future.

Because premia is the direct object of dostaniemy, so it has to be in the accusative case.

• nominative: premia
• accusative: premię

This happens with many feminine singular nouns ending in -a:

• książka → książkę
• kawa → kawę
• premia → premię

So:

• dostaniemy premię = we will get a bonus

Because adjectives in Polish must agree with the noun in gender, number, and case.

Here:

• regał is masculine singular
• firanki is feminine plural

So the adjectives change accordingly:

• większy regał = masculine singular
• nowe firanki = plural

Also, both noun phrases are direct objects after kupię, so they are in the accusative. For these particular nouns, the accusative happens to look the same as the nominative:

• większy regał (same form as nominative)
• nowe firanki (same form as nominative plural)

The preposition do takes the genitive case.

So:

• salon → salonu
• sypialnia → sypialni

In this sentence, do salonu and do sypialni mean something like for the living room and for the bedroom. With furniture, curtains, decorations, and similar items, Polish often uses do + genitive to show what room something is intended for.

Examples:

• stół do kuchni = a table for the kitchen
• kanapa do salonu = a sofa for the living room
• firanki do sypialni = curtains for the bedroom

Yes, firanka exists in the singular, but in real usage firanki is very common because people often talk about curtains as a set rather than as one single item.

So:

• firanka = one net curtain
• firanki = curtains / a set of net curtains

In this sentence, nowe firanki sounds natural because you would usually buy more than one for a bedroom window, or think of them as a pair/set.

Yes. Jeśli and jeżeli both mean if.

• Jeśli dostaniemy premię...
• Jeżeli dostaniemy premię...

Both are correct. In many situations, jeśli sounds a bit more common and natural in everyday speech because it is shorter.

You might also hear jak in very informal speech, but for learners, jeśli and jeżeli are the safest choices.

Because Polish normally uses a comma to separate a subordinate clause from the main clause, and jeśli introduces a subordinate clause.

So:

• Jeśli dostaniemy premię, kupię...

This is standard Polish punctuation, similar to English If we get a bonus, I’ll buy...

Not completely. Polish word order is fairly flexible, because endings carry a lot of grammatical information. The sentence as given is neutral and natural:

• Jeśli dostaniemy premię, kupię większy regał do salonu i nowe firanki do sypialni.

But other orders are possible for emphasis, for example:

• Jeśli dostaniemy premię, większy regał do salonu kupię i nowe firanki do sypialni.

That said, the original version is the most straightforward. For learners, it is a very good default pattern: condition first, then main action.

Because większy is the normal comparative form of duży and means bigger / larger.

• duży = big
• większy = bigger

Polish usually prefers the simple comparative form when it exists, just like English prefers bigger over more big.

So:

• większy regał = a bigger bookcase/shelving unit

Using bardziej duży would sound unnatural here.