Fosforlu kalemim masada değil; galiba davetlilerden biri yanlışlıkla çantasına koymuş.

Fosforlu kalem is the usual Turkish expression for a highlighter.

• fosforlu = fluorescent / phosphorescent-looking
• kalem = pen, pencil, writing tool
• kalemim = my pen/my highlighter

The -im ending is the 1st person singular possessive suffix, meaning my.

So:

• kalem = pen
• kalemim = my pen

In this sentence, fosforlu kalemim means my highlighter.

Because this is not a normal action verb sentence like gitmiyor (isn’t going) or yapmadı (didn’t do).

Here, the idea is simply:

• My highlighter is not on the table.

In Turkish, present-tense to be is often not expressed as a separate word. So:

• masada = on/at the table
• değil = not

Together, masada değil means is not on the table.

So Turkish is literally structured more like:

• My highlighter table-on not.

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

Literally, masa-da is table + locative suffix, meaning in/on/at the table, depending on context.

The suffix -da / -de marks location.

• masa = table
• masada = at/on the table

English chooses the preposition based on natural usage. With table, English usually says on the table, even though Turkish just uses the locative ending.

So in this sentence, masada değil is best understood as isn’t on the table.

Galiba means something like:

• probably
• I guess
• apparently
• I suppose

It shows that the speaker is not fully certain. They are making a guess or inference.

So:

• galiba davetlilerden biri... = probably one of the guests...
• I guess one of the guests...

It makes the sentence sound less definite and more speculative.

This is a very common Turkish pattern:

• X'lerden biri = one of the Xs

Breakdown:

• davetli = invited person / guest
• davetliler = guests / invitees
• davetlilerden = from among the guests
• biri = one of them / one person

So:

• davetlilerden biri = one of the guests

The -den / -dan ending here is the ablative suffix, and in this pattern it gives the sense of out of / from among.

Yanlışlıkla means:

• by mistake
• accidentally

It comes from:

• yanlış = wrong, incorrect
• yanlışlık = mistake
• yanlışlıkla = with/by means of a mistake → by mistake

So in the sentence:

• yanlışlıkla çantasına koymuş = accidentally put it into his/her bag

It is functioning as an adverb, describing how the action happened.

Çantasına means into/to his/her bag.

Breakdown:

• çanta = bag
• çantası = his/her bag
• çantasına = to/into his/her bag

The final part -na is the dative ending, used here because the bag is the destination of the action: the person put something into the bag.

The owner of the bag is understood to be the one guest mentioned by davetlilerden biri.

So:

• davetlilerden biri ... çantasına koymuş = one of the guests put it into his/her bag

In natural English, you might translate that as their bag if you want to avoid choosing his or her.

Because çantaya means to a/the bag, while çantasına means to his/her bag.

Compare:

• çantaya = to the bag / into a bag
• çantasına = to his/her bag / into his/her bag

The sentence is talking about one of the guests accidentally putting the highlighter into their own bag, so the possessive ending is needed.

This is an important difference in Turkish, because possession is usually shown directly on the noun.

This is one of the most important grammar points in the sentence.

• koydu = put (simple past, more direct/definite)
• koymuş = apparently put / must have put / seems to have put

The ending -muş / -miş / -mış / -müş often shows that the speaker:

• did not directly witness the action, or
• is inferring it from evidence, or
• is reporting it indirectly

That fits perfectly with galiba.

So the speaker is not saying:

• I saw a guest put it in the bag.

They are saying something more like:

• I guess one of the guests must have put it in their bag.

So galiba + koymuş both create a tone of inference and uncertainty.

The object is omitted because it is already clear from context.

The missing thing is the highlighter.

Turkish very often leaves out words that are obvious. So instead of saying:

• Galiba davetlilerden biri fosforlu kalemimi yanlışlıkla çantasına koymuş.

the speaker simply says:

• Galiba davetlilerden biri yanlışlıkla çantasına koymuş.

English usually needs the object more often, but Turkish can omit it when it is understood.

If you wanted to make it explicit, you could say:

• Galiba davetlilerden biri fosforlu kalemimi yanlışlıkla çantasına koymuş.
• Galiba davetlilerden biri onu yanlışlıkla çantasına koymuş.

Because Turkish normally prefers verb-final word order.

A very common basic pattern is:

• subject + other information + verb

So the second clause is arranged roughly like this:

• galiba = probably
• davetlilerden biri = one of the guests
• yanlışlıkla = by mistake
• çantasına = into his/her bag
• koymuş = apparently put

That final position for the verb is very normal in Turkish.

So even though English might say:

• One of the guests probably put it in their bag by mistake,

Turkish naturally says something closer to:

• Probably one of the guests by mistake into their bag put.

That is standard Turkish structure.