Öğretmen cümleyi tahtaya yazınca ben de onu deftere geçirdim.

Yazınca comes from yazmak (to write) + -ınca.

This suffix usually means when, once, or after in the sense of when X happened, Y happened.

• yazmak → yazınca = when/once [someone] wrote
• In this sentence: Öğretmen cümleyi tahtaya yazınca... = When the teacher wrote the sentence on the board...

A few things to notice:

• -ınca / -ince / -unca / -ünce changes according to vowel harmony.
• It creates a time clause.
• The subject does not have to be repeated inside the clause if it is already clear.

Examples:

• Eve gelince beni ara. = Call me when you get home.
• Bitirince çıkalım. = Let’s leave after finishing.

Because cümleyi has the accusative case ending, which marks a specific direct object.

• cümle = a sentence / sentence
• cümleyi = the sentence (as a specific object)

Here the teacher is writing a particular sentence, not just any sentence, so Turkish uses the accusative:

• Öğretmen cümleyi yazdı. = The teacher wrote the sentence.

This is very common in Turkish:

• kitap = book
• kitabı = the book (as a definite object)

Since cümle ends in a vowel, the accusative uses a buffer consonant:

• cümle + -(y)i → cümleyi

Tahtaya is tahta (board, blackboard, whiteboard) + the dative case ending -a / -e, which often means to, onto, or toward.

So:

• tahta = board
• tahtaya = to the board / onto the board

In this sentence, the teacher writes the sentence onto the board, so the dative is used.

Because tahta ends in a vowel, Turkish adds the buffer consonant y:

• tahta + -a → tahtaya

Compare:

• masaya koydum = I put it on the table / onto the table
• duvara astı = He hung it on the wall

No, this de is not the locative ending.

In ben de, the word de is a separate particle meaning too, also, or as well.

So:

• ben de = I too / I also

In the sentence, it gives the sense:

• When the teacher wrote the sentence on the board, I also copied it into my notebook.

Important difference:

• de as a separate word = too/also
  • Ben de geldim. = I came too.
• -de attached to a noun = in/on/at
  • Evdeyim. = I am at home.

A very common learner point:
If it means also/too, it is written separately.

Yes, Turkish often drops subject pronouns because the verb ending already shows the subject.

• geçirdim already means I copied / I wrote down
• So Ben de onu deftere geçirdim could become simply Ben de is somewhat optional, and even Onu deftere geçirdim may be enough in context.

Why include ben here?

Usually for emphasis, contrast, or clarity:

• The teacher wrote it on the board, and I also copied it down.

So ben de feels natural because it highlights I too.

Because onu is the accusative form of o.

• o = he / she / it / that
• onu = him / her / it / that (as a direct object)

Here onu refers back to cümleyi (the sentence), and it is the thing being copied, so it must be in the object form.

So:

• onu deftere geçirdim = I copied it into the notebook.

A useful mini-chart:

• o = he/she/it/that
• onu = him/her/it/that (object)
• ona = to him/her/it
• onda = on him/her/it, in him/her/it, at his/her/its place
• ondan = from him/her/it

Notice that pronouns often use n before case endings:

• o + nu → onu

Deftere is defter (notebook) + the dative ending -e, so it means to the notebook / into the notebook.

• defter = notebook
• deftere = into the notebook

The phrase deftere geçirmek is a common expression meaning to copy/write something into a notebook.

Why not defteri?

• defteri would be accusative: the notebook as a direct object.
• But here the notebook is the destination of the writing, not the thing being acted on directly.

So:

• onu deftere geçirdim = I transferred/copied it into the notebook

By itself, geçirmek has several meanings depending on context, and yes, it is related to the idea of causing something to pass / moving something over.

Here, in the expression deftere geçirmek, it means:

• to copy down
• to write into a notebook
• to transfer into writing

So onu deftere geçirdim means something like:

• I copied it into my notebook
• I wrote it down in my notebook

This is a good example of how Turkish verbs can have idiomatic meanings in common combinations.

Some other meanings of geçirmek in different contexts:

• zor zamanlar geçirmek = to go through difficult times
• aklından geçirmek = to think over, run through one’s mind
• temize geçirmek = to copy neatly/fair-copy

Geçirdim can be broken down like this:

• geçir- = the verb stem
• -di = definite past tense
• -m = first person singular (I)

So:

• geçirdim = I copied / I wrote down / I transferred

The -di past is often called the definite past or direct past. It is used for actions the speaker presents as completed facts.

Examples:

• yazdım = I wrote
• okudum = I read
• geçirdim = I copied / transferred

Yes, Turkish word order is fairly flexible, but there is a very common neutral pattern.

A useful way to see this sentence is:

• Öğretmen cümleyi tahtaya yazınca = when the teacher wrote the sentence on the board
• ben de onu deftere geçirdim = I also copied it into the notebook

Turkish often puts the verb at the end of each clause:

• subordinate clause: yazınca
• main clause: geçirdim

That final-verb pattern is very typical.

Within the sentence:

• Öğretmen = subject
• cümleyi = direct object
• tahtaya = destination/location target
• yazınca = subordinate-clause verb
• ben de = main-clause subject + also
• onu = direct object
• deftere = destination
• geçirdim = main verb

Some parts could move for emphasis, but the version given sounds natural and standard.

Yes. Onu refers back to cümleyi.

This is very natural in Turkish: once a noun has been introduced, the next clause can refer to it with a pronoun.

So the structure is:

• The teacher wrote the sentence
• I copied it

Because cümle is not a human noun, English uses it, and Turkish uses onu.

This kind of reference is very common:

• Kitabı aldım. Onu masaya koydum.
  • I took the book. I put it on the table.

Yes. Even if the basic meaning is fixed, Turkish often allows a few natural English renderings.

This sentence could be translated as things like:

• When the teacher wrote the sentence on the board, I copied it into my notebook.
• After the teacher wrote the sentence on the board, I wrote it down in my notebook.
• Once the teacher had written the sentence on the board, I copied it into my notebook.

The difference is mostly style, not grammar.

A useful nuance:

• yazınca can sound like when or once/after, depending on context.
• deftere geçirmek often sounds best in English as write down or copy into my notebook rather than a very literal pass into the notebook.