Doğum günü için balonları şişirirsen iyi olur.

İçin means for. So doğum günü için means for the birthday or for the birthday party.

In this sentence, it gives the purpose of the action: the balloons are being inflated for the birthday.

Doğum günü is the normal Turkish expression for birthday.

Literally:

• doğum = birth
• gün = day
• doğum günü = birthday

This is a common Turkish noun phrase, so learners should treat it as a fixed expression.

The -ı marks the definite direct object.

• balonlar = balloons
• balonları = the balloons

In Turkish, when the object is specific or known, it usually takes the accusative ending. So balonları şişirirsen means if you blow up the balloons, not just some balloons in a general sense.

Compare:

• Balon şişirirsen = if you blow up balloons / if you do balloon-inflating
• Balonları şişirirsen = if you blow up the balloons

Şişirirsen means if you blow up / if you inflate.

It comes from şişirmek, which means to inflate, to blow up, to make swell.

The form breaks down like this:

• şişir- = inflate
• -ir- = aorist/habitual stem
• -sen = if you / you in this conditional form

So şişirirsen literally means something like if you inflate.

Because şişmek and şişirmek are different verbs.

• şişmek = to swell, to become inflated
• şişirmek = to inflate something, to blow something up

A balloon does şişmek by itself, but a person şişirir the balloon.

So:

• Balon şişti = The balloon inflated / swelled up
• Balonu şişirdim = I inflated the balloon

In your sentence, the person is doing the action to the balloons, so Turkish uses şişirmek.

Literally, iyi olur means it becomes good or it would be good.

Naturally in English, it often means:

• that would be good
• it would be nice
• it’d be better
• it would be a good idea

In this sentence, iyi olur makes the whole thing sound like a suggestion or polite request rather than a direct command.

Not exactly. It is usually understood as a soft suggestion or polite request.

• Balonları şişir. = Blow up the balloons.
  This is a direct command.

• Balonları şişirirsen iyi olur. = It would be good if you blew up the balloons.
  This is softer and less direct.

So Turkish often uses this kind of structure to sound more polite or less forceful.

Because Turkish usually leaves out subject pronouns when the verb already shows the person.

In şişirirsen, the ending already tells you the subject is you (singular).

So sen is not necessary.

You could say sen balonları şişirirsen iyi olur, but it would usually add emphasis, and in many contexts it may sound unnecessary.

Turkish often uses an if-clause + iyi olur to make suggestions in a gentle way.

So the structure is:

[if you do X] + [it would be good]

In this sentence:

• balonları şişirirsen = if you blow up the balloons
• iyi olur = that would be good

This does not always sound like a real condition in English. Very often, it functions more like:

• You should blow up the balloons
• Could you blow up the balloons?
• It’d be good if you blew up the balloons

It is based on the aorist form plus the conditional ending.

For learners, the easiest way to think about it is:

• şişirir = blows up / would blow up / does inflate
• şişirirsen = if you blow up / if you were to blow up

In Turkish, the aorist is often used in general statements, suggestions, and conditional sentences. So even though English may use blew up or would blow up, Turkish uses this form very naturally here.

Yes. Turkish word order is flexible, though some versions sound more neutral than others.

The original sentence:

• Doğum günü için balonları şişirirsen iyi olur.

This is a natural order: purpose first, then object, then verb phrase.

You could also hear:

• Balonları doğum günü için şişirirsen iyi olur.

That still means the same thing, but the emphasis shifts slightly.

In Turkish, word order often changes to highlight different parts of the sentence rather than to change the basic meaning.

The ğ in Turkish is usually not pronounced like a hard consonant in English.

In doğum, it does not sound like a strong g. Instead, it usually lengthens or smooths the surrounding vowels. So doğum sounds roughly like do-um, with a soft glide between the vowels.

Also:

• ş is pronounced like sh in shoe
• so şişirirsen begins with a shi- sound

A rough pronunciation guide:

• doğum günü ≈ doh-oom gyu-nyu
• şişirirsen ≈ shi-shee-reer-sen

That is only approximate, but it helps an English speaker get close.