İnanılmaz; o saçma cevap doğruymuş.

Here İnanılmaz is being used as a standalone reaction, like Unbelievable! or Incredible!

Literally, it comes from a form meaning not believable, but in natural usage it often works as an exclamation expressing surprise.

The semicolon separates the speaker’s reaction from the explanation that follows.

So the structure is roughly:

• İnanılmaz = the reaction
• o saçma cevap doğruymuş = the reason for that reaction

In everyday writing, you might also see a comma, dash, or even just a full stop instead. The semicolon is a stylistic punctuation choice.

Here o means that.

A very useful rule is:

• o by itself can mean he, she, or it
• o before a noun usually means that

So:

• o = he/she/it
• o cevap = that answer
• o saçma cevap = that ridiculous answer

Because Turkish adjectives normally come before the noun they describe, just like in English.

So:

• saçma cevap = ridiculous answer
• doğru cevap = correct answer

Also, Turkish adjectives do not change form to agree with the noun. saçma stays saçma.

No. Turkish does not have direct equivalents of English a and the in the same way.

Definiteness is often understood from context, or shown with words like bu (this) and o (that).

So o saçma cevap already clearly means something like that ridiculous answer.

Saçma means nonsense, absurd, ridiculous, or silly, depending on context.

In this sentence, it describes the answer as something the speaker considers obviously bad, foolish, or absurd.

So o saçma cevap is not just that answer, but that ridiculous / absurd answer.

Turkish often does not use a separate word for to be in sentences like this.

Instead, the predicate itself carries the meaning of is/was/seems/was apparently, often through suffixes.

Here:

• doğru = correct
• doğruymuş = was apparently correct / turns out it was correct

So the idea of was is built into doğruymuş rather than expressed as a separate word.

Doğruymuş is made of:

• doğru = correct / right
• -muş = a suffix showing reported information, later discovery, or surprise
• -y- = a linking sound

So doğruymuş means something like:

• it was apparently correct
• it turns out it was correct
• so it was correct after all

In this sentence, it strongly suggests surprise.

The y is a buffer sound.

Turkish often inserts y when a suffix is attached to a word ending in a vowel, especially in forms related to the copula.

Since doğru ends in a vowel, the language uses:

• doğru + y + muş → doğruymuş

That makes pronunciation smoother.

Because of vowel harmony.

This suffix has four forms:

• -mış
• -miş
• -muş
• -müş

The choice depends on the last vowel of the word before it. In doğru, the last vowel is u, so the correct form is -muş.

That is why it becomes doğruymuş.

This is one of the most important parts of the sentence.

The -muş form often shows that the speaker:

• learned the information from someone else
• discovered it afterward
• is expressing surprise
• is saying apparently, it turns out, or so...

So doğruymuş is different from a plain factual was correct. It has a feeling like:

• Apparently it was correct
• Turns out it was correct
• So that ridiculous answer was actually correct

That nuance fits very well with İnanılmaz.

Because the subject is third person singular, and in Turkish that form often has no extra personal ending in this kind of sentence.

So:

• doğruymuşum = apparently I was right
• doğruymuşsun = apparently you were right
• doğruymuş = apparently he/she/it was right

Since o saçma cevap is a singular thing, doğruymuş is enough.

This is a very useful comparison.

• doğruydu = it was correct
  This is a straightforward past statement.

• doğruymuş = it was apparently correct / it turns out it was correct
  This adds indirectness, discovery, or surprise.

So in your sentence, doğruymuş is better because the speaker is reacting with surprise: Unbelievable; that ridiculous answer turns out to have been correct.

The word order is quite normal.

• o saçma cevap = subject
• doğruymuş = predicate

So the basic structure is:

That ridiculous answer + was apparently correct

Turkish word order is flexible, but this version is neutral and natural. A different order could be used for emphasis, but the sentence as given is very standard.