Onay kutusunu işaretledim ve küçük bir espri yapınca herkes güldü.

Because it is the direct object of işaretledim and it is a specific/definite one.

In Turkish, a specific direct object usually takes the accusative ending. Here the base expression is onay kutusu = checkbox.

Then:

• onay kutusu → checkbox
• onay kutusunu → the checkbox / that checkbox, as the object

The ending looks a little unusual because kutusu already contains a compound/possessive-type ending, so Turkish adds a buffer -n- before the accusative:

• kutu
• kutusu
• kutusunu

So onay kutusunu işaretledim means I checked/ticked the checkbox.

Onay kutusu is the normal Turkish term for checkbox.

It is made from:

• onay = approval / confirmation
• kutu = box

Together, onay kutusu literally means something like approval box, but in actual use it means checkbox.

This is a very common Turkish noun-compound pattern. The second noun often gets an -(s)I ending in these compounds:

• kutu → box
• onay kutusu → checkbox

So even though English just says checkbox, Turkish expresses it as a compound noun.

İşaretledim can be broken down like this:

• işaretle- = to mark / to tick / to indicate
• -di = past tense
• -m = I

So:

• işaretledim = I marked / I checked / I ticked

This is a very common Turkish verb pattern:

• verb stem + past tense + person ending

For example:

• geldim = I came
• yaptım = I did
• işaretledim = I marked

Because Turkish often drops subject pronouns when the verb already shows the person clearly.

In işaretledim, the ending -m already means I, so ben is not necessary.

So Turkish prefers:

• Onay kutusunu işaretledim.

rather than:

• Ben onay kutusunu işaretledim.

You can still say ben if you want emphasis or contrast, for example:

• Ben işaretledim, o işaretlemedi.
  I checked it, he/she didn’t.

Yapınca comes from:

• yap- = do / make
• -ınca = when / once / after doing

So yapınca means something like:

• when [someone] did/made
• once [someone] did/made
• after [someone] did/made

In this sentence, küçük bir espri yapınca means when I made a little joke.

The suffix has different vowel-harmony forms:

• -ınca
• -ince
• -unca
• -ünce

Examples:

• gelince = when he/she comes / when he/she came
• bitirince = when he/she finishes / finished
• okuyunca = when he/she reads / read

The exact time reference is understood from the context and the main verb.

Because yapınca itself does not show person. The subject is understood from context.

In this sentence, the first clause is işaretledim = I checked, so the natural reading is that the same speaker also made the joke:

• I checked the checkbox, and when I made a little joke, everyone laughed.

If Turkish wants to make the subject explicit, it can:

• Ben küçük bir espri yapınca herkes güldü.
• Ali küçük bir espri yapınca herkes güldü.

So yapınca does not itself mean when I made or when he made. Context tells you whose action it is.

Here bir works like the English indefinite article a.

So:

• küçük = small / little
• bir = a
• espri = joke / witty remark

Together:

• küçük bir espri = a little joke

Turkish often uses bir in places where English uses a/an:

• bir kitap = a book
• bir adam = a man
• küçük bir sorun = a small problem

Without bir, the phrase can sound more general or less natural in many contexts.

Yes. Espri yapmak is a very natural expression meaning:

• to joke
• to make a joke
• to make a witty remark

It is built from:

• espri = joke / wit
• yapmak = to do / make

This kind of noun + yapmak structure is extremely common in Turkish.

You may also hear:

• şaka yapmak = to joke / to play a joke

A rough difference is:

• espri often feels more like a witty comment
• şaka can be a joke, prank, or playful remark

In this sentence, espri yapmak fits well because it suggests making a funny remark.

Yes, herkes means everyone, but grammatically it usually behaves as singular in Turkish.

So Turkish says:

• herkes güldü = everyone laughed

not:

• herkes güldüler

This is similar to English, where we also say everyone laughs rather than everyone laugh.

So the structure is completely normal:

• herkes = everyone
• güldü = laughed

Yes. The verb gülmek can mean both to laugh and to smile, depending on context.

But in this sentence, laughed is clearly the right meaning because of küçük bir espri and herkes:

• someone made a joke
• everyone reacted

That strongly points to everyone laughed, not just everyone smiled.

So context decides which English word fits best.

Because Turkish normally puts the verb near the end, and subordinate parts often come before the main verb.

A very literal feel of the sentence is something like:

• The checkbox I-checked and a little joke when-I-made everyone laughed.

That sounds strange in English, but it is normal in Turkish.

A few key points:

• onay kutusunu comes before işaretledim
• küçük bir espri yapınca comes before herkes güldü
• ve simply links the two parts as and

So the sentence structure is very Turkish even though the meaning is straightforward in English.