Ben mektubu yolladım ama adres hatalıydı.

Yes, ben can be omitted.

Turkish verbs usually show the subject clearly through their endings. In yolladım, the ending -ım already means I, so Yolladım ama adres hatalıydı is also completely natural.

Using ben often adds:

• emphasis
• contrast
• or a clearer starting point for the sentence

So:

• Ben mektubu yolladım. = I sent the letter.
• Mektubu yolladım. = also I sent the letter.

In this sentence, ben may slightly emphasize that I did send it, but there was another problem: the address was wrong.

Because mektubu is the definite direct object.

In Turkish, when a direct object is specific/definite, it usually takes the accusative ending. Here, the letter is a specific letter, so:

• mektup = a letter / letter
• mektubu = the letter

There is also a sound change:

• mektup
  • -u → mektubu

The final p changes to b when a vowel-initial suffix is added. This is a common consonant change in Turkish.

Compare:

• Mektup yolladım. = I sent a letter.
• Mektubu yolladım. = I sent the letter.

Yolladım breaks down like this:

• yolla- = send
• -dı = past tense
• -m / -ım = I

So:

• yolla-dı-m = I sent

Because of vowel harmony, the tense appears here as -dı, and the first-person ending appears as -ım.

A very common synonym is gönderdim:

• Ben mektubu gönderdim.
• Ben mektubu yolladım.

Both mean I sent the letter, though yollamak can sound a bit more everyday or conversational in many contexts.

Ama means but.

It connects two clauses that contrast with each other:

• Ben mektubu yolladım = I sent the letter
• ama adres hatalıydı = but the address was wrong

So the idea is: I did send the letter, but there was a problem: the address was incorrect.

Other Turkish words that can mean but include:

• fakat
• ancak

But ama is very common and natural in everyday speech.

In Turkish, the verb to be is often expressed differently from English.

Here, hatalı means incorrect / faulty, and the past meaning was is added with the copular past form:

• hatalıydı = was incorrect

This comes from:

• hatalı
  • idi → hatalıydı

The y appears as a linking sound between vowels.

So:

• Adres hatalı. = The address is incorrect.
• Adres hatalıydı. = The address was incorrect.

Turkish often attaches this kind of was/were meaning directly to the predicate instead of using a separate standalone verb like English does.

Because adres is the subject of the second clause:

• adres hatalıydı = the address was incorrect

Subjects in Turkish are often in the basic form, with no extra case ending.

Compare:

• Adres hatalıydı. = The address was wrong.
  (adres = subject)
• Adresi yazdım. = I wrote the address.
  (adresi = object, with accusative)

So in your sentence, adres is not receiving the action; it is what the statement is about.

Hatalı means something like:

• incorrect
• faulty
• containing an error

For an address, hatalı works very naturally: the address had a mistake in it.

Yanlış also means wrong / incorrect, and in many situations it can be similar:

• Adres yanlıştı.
• Adres hatalıydı.

Possible nuance:

• yanlış = wrong
• hatalı = erroneous / mistaken / faulty

In many everyday contexts, both are possible, but hatalı can sound a little more like there was an error in it.

The first clause follows a very common Turkish pattern:

• Ben = subject
• mektubu = object
• yolladım = verb

So it is basically Subject + Object + Verb, which is a very typical Turkish order.

The second clause:

• adres = subject
• hatalıydı = predicate

Turkish is flexible, though, and word order can change for emphasis. For example:

• Mektubu ben yolladım ama adres hatalıydı.

This puts more emphasis on ben: I sent the letter.

So the sentence you have is neutral and standard.

Turkish does not have articles like a and the in the same way English does.

Whether something is understood as a or the depends on:

• context
• word order
• case marking
• shared knowledge

In adres hatalıydı, English naturally says the address was wrong, but Turkish simply says adres hatalıydı.

In the first clause, definiteness is shown more clearly by the accusative:

• mektubu = the letter

So Turkish often expresses definiteness without a separate word like the.

Yes. Turkish has several ways to express this idea. For example:

• Ben mektubu yolladım, fakat adres hatalıydı.
• Ben mektubu yolladım, ancak adres hatalıydı.

These all mean roughly the same thing.

Differences:

• ama = very common, everyday
• fakat = also common, slightly more formal in some contexts
• ancak = can sound a bit more formal or written, depending on context

So ama is probably the most straightforward choice for a learner.

A native speaker would usually say it smoothly, with the stress and rhythm flowing across words rather than pausing after each one.

A few helpful points:

• mektubu is pronounced roughly mek-tu-bu
• yolladım is roughly yol-la-dım
• hatalıydı is roughly ha-ta-LIY-dı

Important detail:

• In hatalıydı, the y is pronounced clearly as a linker.

You do not need to over-separate the endings. It should sound like whole words, not like grammar pieces being read out one by one.

A natural rhythm would be something like:

Ben mektubu yolladım | ama adres hatalıydı.

with a small contrastive pause before ama.