Bu ciddi bir suç.

• Bu = this
• ciddi = serious
• bir = a / an (sometimes also one)
• suç = crime, offense, sometimes fault/blame depending on context

So the sentence means This is a serious crime.

In Turkish, the verb to be is often not stated in simple present-tense sentences like this, especially in the third person.

So Turkish can say:

• Bu ciddi bir suç. = This is a serious crime.

without a separate word for is.

A more formal or emphatic version is:

• Bu ciddi bir suçtur.

That -tur ending adds a more formal is sense, but in everyday speech it is usually omitted.

Here, bu means this by itself.

So the structure is:

• Bu = this
• ciddi bir suç = a serious crime

In other words, the sentence is literally structured like:

• This [is] a serious crime.

If you wanted this serious crime as a noun phrase, you would say:

• bu ciddi suç

But Bu ciddi bir suç. is a complete sentence.

In Turkish, when bir means a/an and there is an adjective, the usual order is:

• adjective + bir + noun

So:

• ciddi bir suç = a serious crime
• büyük bir ev = a big house
• güzel bir kitap = a beautiful book

For that reason, Bu bir ciddi suç sounds unnatural in standard Turkish.

Here, bir means a/an, not one.

So:

• ciddi bir suç = a serious crime

Turkish bir can also mean one, but the meaning depends on context.

For example:

• bir suç = a crime
• bir suç değil, iki suç = not one crime, but two crimes

In your sentence, it is just the ordinary indefinite article meaning a.

Usually, for this meaning, bir should stay.

• Bu ciddi bir suç. = This is a serious crime.

If you say Bu ciddi suç, it will usually not sound like the normal way to say This is a serious crime. It can sound incomplete, or it may be understood more like a noun phrase such as this serious crime, depending on context.

So for learners, the safe and natural sentence is:

• Bu ciddi bir suç.

A rough pronunciation guide:

• Bu ≈ boo
• ciddi ≈ jid-dee
• bir ≈ beer (with a quick tapped r)
• suç ≈ sooch

A few helpful spelling notes:

• Turkish c is pronounced like English j
• Turkish ç is pronounced like English ch

So ciddi starts with a j sound, and suç ends with a ch sound.

Suç often means crime or offense, especially in legal or moral contexts.

Examples:

• Bu bir suç. = This is a crime.
• suç işlemek = to commit a crime

But suç can also mean fault or blame in some expressions:

• Suç bende. = It’s my fault.
• Onun suçu değil. = It’s not his/her fault.

In Bu ciddi bir suç., the meaning is clearly crime/offense.

The pattern is:

• Bu
  • predicate noun phrase

More specifically:

• Bu = subject-like element (this)
• ciddi bir suç = predicate noun phrase (a serious crime)

So Turkish is building an equational sentence:

• This = a serious crime

This is a very common Turkish pattern:

• Bu bir sorun. = This is a problem.
• Bu iyi bir fikir. = This is a good idea.
• Bu önemli bir konu. = This is an important topic.

Yes.

• Bu bir suç. = This is a crime.
• Bu ciddi bir suç. = This is a serious crime.

The second sentence simply adds ciddi to make the meaning stronger and more specific.

You can do this very easily:

• Bu ciddi bir suç mu? = Is this a serious crime?
• Bu ciddi bir suç değil. = This is not a serious crime.

So:

• mu makes a yes/no question
• değil makes it negative

These are very common patterns in Turkish nominal sentences.