Learn Hardcore Turkish: Doğum gününde balonları şişirirsen iyi olur. - It would be good if you inflate the balloons for the birthday.

şişir-: root “inflate” (causative of şiş- “to swell”)
-ir: aorist/general tense
-se: conditional marker (-sa/-se)
-n: 2nd person singular So şişir-ir-se-n ≈ “if you inflate.” The aorist + conditional is the default way to talk about a real, future-oriented condition and is commonly used to make polite suggestions or instructions.

Can I say şişirsen instead of şişirirsen? What’s the difference?

şişirirsen is the neutral, straightforward conditional “if you inflate.”
şişirsen (no aorist) leans toward a tentative suggestion/wish, like “if you were to inflate / how about inflating.” It’s a bit more coaxing. Both are acceptable; the original with -ir- is the more standard conditional for plans/instructions.

Is Doğum gününde “on your birthday” or “on his/her birthday”? How do I say whose birthday it is?

It’s ambiguous and must be resolved by context. Orthographically, both 2nd person and 3rd person look the same:

2nd person: doğum günün + de → doğum gününde (on your birthday)
3rd person: doğum günü + nde → doğum gününde (on his/her birthday) To clarify, add the possessor:
Benim doğum günümde (on my birthday)
Senin doğum gününde (on your birthday)
Onun/Ali’nin doğum gününde (on his/her / Ali’s birthday)

Where does the “n” in gününde come from?

It’s a buffer consonant used when a possessed noun takes a case suffix.

3rd person: günü + de → gününde
2nd person: günün + de → gününde This -n- prevents vowel clash. You see the same in forms like evi + de → evinde.

Why is it balonları (accusative)? Could I say balonlar or just balon?

In Turkish, a specific/definite direct object takes the accusative. Here, the balloons are a known set (the party balloons), so balon-lar-ı is used. If you mean balloons in a non-specific, generic way, you drop the accusative:

Generic/indefinite: balon şişirirsen (“if you inflate balloons [some balloons]”) Using balonlar without accusative is possible but less typical than the bare singular for an indefinite object. With a definite set, keep balonları.

What does iyi olur add? Is it a kind of polite request?

Yes. iyi olur (“it would be good”) frames the request as a consequence of the condition, which softens it. It’s less direct than an imperative and sounds like guidance/expectation. Compare:

Balonları şişirir misin? A direct polite request (“Could you inflate the balloons?”)
Balonları şişirirsen iyi olur. A suggestion/instruction (“It’d be good if you inflated the balloons.”)

How can I make it even softer or more polite?

Doğum gününde balonları şişirirsen çok sevinirim. (I’d be very happy if…)
Doğum gününde balonları şişirebilir misin lütfen? (Could you please…)
Doğum gününde balonları şişirsen iyi olurdu. (would be good… — extra soft/less direct)
Doğum gününde balonları şişirmen çok iyi olur. (using a nominalized subject)

How do I say it as a counterfactual or mild reproach: “It would have been good if you had inflated the balloons (but you didn’t)”?

Use past conditional + iyi olurdu:

Doğum gününde balonları şişirseydin, iyi olurdu.

Can I rearrange the word order?

Yes. Keep iyi olur at the end; move the other parts for emphasis:

Balonları doğum gününde şişirirsen iyi olur.
Doğum gününde, balonları şişirirsen iyi olur. Word order is flexible, but don’t split iyi olur.

Is şişirmek the right verb for balloons? How is it different from üflemek and şişmek?

şişirmek: to inflate something (correct for balloons), e.g., balonları şişirmek.
üflemek: to blow (air), e.g., muma üflemek (blow a candle). Not used to mean “inflate balloons.”
şişmek: to swell (intransitive), e.g., balon şişti (the balloon swelled/was inflated).

Is Doğum gününde balonları şişirmen iyi olur correct? How does it differ?

Yes. şişir-men is a nominalized verb (+ 2sg poss), literally “your inflating (the balloons).” The sentence treats that act as the subject: “Your inflating the balloons on the birthday would be good.” It feels slightly more matter-of-fact/planned than the conditional.

Can I use the future conditional, like şişireceksen iyi olur?

Grammatically yes, but it shifts the meaning to “if you’re going to inflate them (if that’s your plan), that’d be good.” For a neutral suggestion/request, the aorist conditional şişirirsen is the default. You can also say şişirebilirsen iyi olur (“if you can inflate them…”), which adds a nuance of ability.

Why not şişerirsen?

Because the causative root is şişir-, not şişer-. The pattern is:

şiş- (to swell) → causative şişir- (to make swell = to inflate) Hence the correct conditional is şişirirsen.