Yeni sekmeyi açar açmaz yazı tipini ve puntoyu değiştirdim.

It means as soon as (I) opened or the moment (I) opened.

So:

• Yeni sekmeyi açar açmaz... = As soon as I opened the new tab...
• It gives a strong sense of immediacy: one action happened immediately after the other.

It is stronger and more immediate than a more general when I opened.

This is a common Turkish pattern:

verb stem + -(A)r ... same verb + -maz/-mez

In this sentence:

• açar = opens / opening
• açmaz = literally does not open, but in this fixed pattern it does not mean negation in the normal sense

Together, açar açmaz means as soon as opening/opened.

Other examples:

• gelir gelmez = as soon as he/she came
• duyar duymaz = as soon as he/she heard
• biter bitmez = as soon as it ends/ended

This structure is idiomatic, so it is best learned as a set pattern meaning immediately after doing something.

Because sekmeyi is the definite direct object form of sekme.

• sekme = a tab
• sekmeyi = the tab

The ending -(y)i here is the accusative marker, which is often used when the object is specific or definite.

So:

• Yeni sekme açtım = I opened a new tab
• Yeni sekmeyi açtım = I opened the new tab / that specific new tab

In this sentence, the speaker is referring to a specific tab they opened, so the accusative makes sense.

Because yazı tipi is a noun compound in Turkish.

• yazı = writing/text
• tip = type
• yazı tipi = font / typeface

In this kind of compound, the second noun already has a built-in compound ending: tip-i.

Then, when you add the accusative, it becomes:

• yazı tipi → yazı tipini

So the -ni is not random:

• -i = part of the compound
• -ni = accusative added after that compound form

This is very common in Turkish noun compounds:

• kapı kolu → kapı kolunu
• telefon numarası → telefon numarasını
• yazı tipi → yazı tipini

Punto is also a direct object here, and it is specific, so it takes the accusative too.

• punto = point size / font size
• puntoyu = the point size / the font size

The -y- is just a buffer consonant used because the noun ends in a vowel.

So:

• punto + u would be awkward
• Turkish uses puntoyu

This is the same thing you see in words like:

• masayı
• arabayı
• odayı

In this sentence:

• yazı tipi = font or typeface
• punto = font size or point size

So yazı tipini ve puntoyu değiştirdim means the speaker changed:

• the font/typeface, and
• the font size

In computer/interface Turkish, both are natural, though depending on context you may also hear expressions like yazı boyutu for font size.

Because değiştirmek means to change something.

Breakdown:

• değiştir- = change, alter
• -di = past tense
• -m = I

So değiştirdim = I changed

This is different from değişmek, which means to change in the sense of to undergo change.

Compare:

• Renk değişti. = The color changed.
• Rengi değiştirdim. = I changed the color.

Here the speaker is actively changing the font and size, so değiştirdim is the correct form.

It is contained in the verb ending.

• değiştirdim = I changed

The final -m shows first person singular, so Turkish does not need a separate subject pronoun unless you want emphasis.

So:

• değiştirdim already means I changed
• ben değiştirdim would mean I changed it, with extra emphasis on I

The subject of açar açmaz is also understood from context as the same person.

Turkish normally puts the verb at the end of the clause.

So the structure is:

• Yeni sekmeyi açar açmaz = as soon as I opened the new tab
• yazı tipini ve puntoyu değiştirdim = I changed the font and the size

In English, we often say:

• As soon as I opened the new tab, I changed the font and the point size.

In Turkish, it is very normal for:

• the time clause to come first
• the main verb to come last

That gives the sentence a very natural Turkish flow.

Yes, but the meaning changes slightly.

• Yeni sekmeyi açınca yazı tipini ve puntoyu değiştirdim.
  = When I opened the new tab, I changed the font and point size.

• Yeni sekmeyi açar açmaz yazı tipini ve puntoyu değiştirdim.
  = As soon as I opened the new tab, I changed the font and point size.

So açınca is more neutral: when/upon opening.
açar açmaz stresses that the second action happened immediately.

Not quite in the same way as an English clause with an explicit subject and tense.

It functions like a subordinate time expression meaning:

• as soon as I opened the new tab

But Turkish does not explicitly mark person there. The person and tense are mainly understood from the main verb and the context.

So even though English needs something like I opened, Turkish can say açar açmaz and let the main clause carry the person and tense information.

That is very normal in Turkish subordinate structures.