Öğretmen beni tebrik etti, ben de arkadaşımı tebrik etmeyi unutmadım.

Because beni is the accusative form of ben (I / me).

• ben = I
• beni = me as a definite direct object

In Öğretmen beni tebrik etti, the teacher is doing the action of congratulating, and me is the person receiving that action, so Turkish uses beni.

This is very common with pronouns:

• ben → beni
• sen → seni
• o → onu
• biz → bizi
• siz → sizi
• onlar → onları

Tebrik etti is a very common Turkish verb construction: a noun + etmek combination.

• tebrik = congratulations / congratulation
• etmek = to do / to make

Together, tebrik etmek means to congratulate.

So:

• tebrik etti = he/she congratulated

This is not unusual in Turkish. Many actions are expressed with a noun plus etmek:

• yardım etmek = to help
• telefon etmek = to phone
• davet etmek = to invite

In your sentence:

• Öğretmen beni tebrik etti = The teacher congratulated me

Etti is the past tense, 3rd person singular form of etmek.

Breakdown:

• et- = verb stem
• -ti = past tense ending
• third person singular often has no extra ending in this tense

So:

• ettim = I did
• ettin = you did
• etti = he/she did
• ettik = we did

Because the subject is öğretmen (the teacher), Turkish uses etti.

Turkish does not have articles like the or a/an.

So öğretmen can mean:

• teacher
• the teacher
• sometimes even a teacher

The exact meaning comes from context.

In this sentence, English naturally uses the teacher, but Turkish just says öğretmen.

Because de here means also / too, and in this meaning it is written separately.

• ben de = I too / I also
• bende = on me / with me / in me depending on context

So these are completely different:

• Ben de geldim. = I came too.
• Anahtar bende. = I have the key. / The key is with me.

In your sentence, ben de shows that the speaker is adding their own action to the previous clause:

• The teacher congratulated me, and I also...

The particle de/da follows vowel harmony.

After words whose last vowel is a front vowel (e, i, ö, ü), you use de. After words whose last vowel is a back vowel (a, ı, o, u), you use da.

Here the word is ben, and its last vowel is e, so:

• ben de

Compare:

• Sen de gel. = You come too.
• O da geldi. = He/she came too.

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

For example:

• unutmadım already means I did not forget

So in theory, the second clause could be shorter. But ben is included here for emphasis or contrast:

• Öğretmen beni tebrik etti, ben de...
• The teacher congratulated me, and I too...

It helps create a nice parallel idea: someone congratulated me, and I also congratulated someone else.

Arkadaşımı breaks down like this:

• arkadaş = friend
• -ım = my
• -ı = accusative ending

So arkadaşımı means my friend as a definite direct object.

Why accusative? Because the friend is the thing/person being congratulated:

• arkadaşımı tebrik etmek = to congratulate my friend

This is an important point: when a noun has a possessive ending like -ım (my), and it is also the direct object, Turkish often adds the accusative after that:

• kitabımı okudum = I read my book
• annemi gördüm = I saw my mother
• arkadaşımı tebrik ettim = I congratulated my friend

Because after unutmak (to forget), Turkish often uses a verbal noun as the object of the verb.

Here:

• tebrik etmek = to congratulate
• tebrik etmeyi = the act of congratulating, in the accusative

Breakdown:

• etme = verbal noun / action noun from etmek
• -yi = accusative ending

So:

• tebrik etmeyi unutmadım = I didn’t forget to congratulate

This is a very common structure:

• gelmeyi unuttum = I forgot to come
• aramayı unutma = don’t forget to call
• kapıyı kapatmayı unuttum = I forgot to close the door

English uses to + verb here, but Turkish often uses this -me/-ma + accusative pattern.

Because Turkish is turning the verb into something noun-like so it can function as the object of unutmadım.

The steps are roughly:

• etmek = to do / to make
• etme = doing / the act of doing
• etmeyi = the act of doing (as a definite object)

So in tebrik etmeyi unutmadım, the thing not forgotten is literally the act of congratulating.

This may feel different from English, but it is very natural in Turkish.

Unutmadım means I did not forget.

Breakdown:

• unut- = forget
• -ma- = negative
• -dı- = past tense
• -m = I

So:

• unuttum = I forgot
• unutmadım = I did not forget

Notice that Turkish builds this into one word, whereas English uses several words.

Turkish word order is often more flexible than English, but the neutral pattern is usually Subject – Object – Verb.

So:

• Öğretmen beni tebrik etti
  • Öğretmen = subject
  • beni = object
  • tebrik etti = verb

And in the second clause:

• ben de arkadaşımı tebrik etmeyi unutmadım
  • ben de = I too
  • arkadaşımı = my friend
  • tebrik etmeyi = congratulating
  • unutmadım = I did not forget

The main verb often comes at the end in Turkish, so English speakers often need time to get used to waiting for the verb.

No, not in this form.

Because arkadaşımı tebrik etmeyi unutmadım clearly means I didn’t forget to congratulate my friend.

The clues are:

• arkadaşımı is the direct object of tebrik etmek
• tebrik etmeyi is the action that was not forgotten
• unutmadım is the main verb

So the structure is:

[my friend] + [to congratulate] + [I didn’t forget]

That makes the friend the one being congratulated, not the one doing the congratulating.

Yes. Tebrik etmek normally takes a direct object in the accusative when the person being congratulated is specific.

Examples:

• Beni tebrik etti. = He/She congratulated me.
• Arkadaşımı tebrik ettim. = I congratulated my friend.
• Öğrencileri tebrik etti. = He/She congratulated the students.

So in your sentence, both beni and arkadaşımı are exactly what you would expect with tebrik etmek.

The comma helps separate the two related clauses:

• Öğretmen beni tebrik etti
• ben de arkadaşımı tebrik etmeyi unutmadım

It makes the sentence easier to read and shows a natural pause. In everyday writing, punctuation in Turkish can sometimes be a little flexible, but here the comma is perfectly natural because the sentence contains two coordinated clauses.

Yes, depending on context, Turkish offers some shorter alternatives. For example, if the subject is obvious, speakers might omit ben:

• Öğretmen beni tebrik etti, arkadaşımı tebrik etmeyi de unutmadım.

Or they might rearrange things slightly for emphasis.

But the given sentence is grammatical and natural, especially if the speaker wants to stress I too:

• Öğretmen beni tebrik etti, ben de...

That contrast is probably why ben de is placed so clearly in the sentence.