No creo que eso sea verdad.

Both creer and pensar can mean to think, but they are not always used in exactly the same way.

• Creo que... often means I believe / I think
• Pienso que... can also mean I think, but it can sound a bit more like in my opinion or I’m of the view that...

In this sentence, No creo que eso sea verdad is a very natural way to say I don’t think that’s true.

Que means that and introduces the second clause:

• No creo = I don’t think
• que eso sea verdad = that that is true

In English, that is often optional:

• I don’t think that’s true
• I don’t think that that is true

In Spanish, que is normally required here.

Because after no creo que, Spanish normally uses the subjunctive.

• es = present indicative of ser
• sea = present subjunctive of ser

Spanish uses the subjunctive after expressions of doubt, denial, uncertainty, or non-belief.
Since No creo que... expresses not believing something, sea is the correct form:

• No creo que eso sea verdad. = I don’t think that’s true.

In normal usage, yes: when creer is negative and followed by another clause, Spanish usually uses the subjunctive.

• Creo que es verdad. = I think it’s true.
• No creo que sea verdad. = I don’t think it’s true.

This contrast is very important:

• affirmative belief → usually indicative
• negative belief / doubt → usually subjunctive

Sea is the first-person singular and third-person singular present subjunctive form of ser.

Here it matches eso, which is grammatically treated as third-person singular:

• que eso sea verdad

You will also see sea with other subjects:

• No creo que él sea médico.
• No creo que ella sea de Madrid.

Eso means that or that thing. It refers to something already mentioned or understood from context.

Spanish has three common demonstratives:

• esto = this
• eso = that
• aquello = that over there / that thing over there

In No creo que eso sea verdad, eso means that thing you said / that idea / that claim.

Sometimes, yes, but the meaning changes slightly.

• esto = this
• eso = that

So:

• No creo que esto sea verdad = I don’t think this is true.
• No creo que eso sea verdad = I don’t think that is true.

Which one sounds right depends on how close the speaker feels the idea is, physically or mentally. In many conversations, eso is the more natural choice for referring to something another person just said.

Because verdad here is being used in the expression ser verdad = to be true.

So:

• Eso es verdad. = That is true.
• No creo que eso sea verdad. = I don’t think that is true.

You do not normally say eso sea la verdad in this context.
La verdad usually means the truth as a noun:

• Quiero saber la verdad. = I want to know the truth.

But ser verdad means to be true.

Because Spanish often drops subject pronouns when they are clear from the verb form.

• creo already means I believe / I think

So (Yo) no creo que eso sea verdad and No creo que eso sea verdad both mean the same thing.

You would add yo only for emphasis, contrast, or clarity:

• Yo no creo que eso sea verdad, pero ella sí.
  = I don’t think that’s true, but she does.

A little, but the standard and most natural order is:

• No creo que eso sea verdad.

You might also hear variations for emphasis, but they are less neutral. For example:

• Eso no creo que sea verdad.

This puts more focus on eso. For learners, the original sentence is the safest and most natural pattern to use.

It is a noun, but in the fixed expression ser verdad, it functions like English to be true.

Literally:

• verdad = truth

But Spanish commonly says:

• Es verdad = It’s true
• No es verdad = It’s not true

So even though verdad is a noun, the whole expression behaves like an English adjective phrase.

Yes, it is grammatical, and it would still take the subjunctive because the main clause is negative.

• No pienso que eso sea verdad.

However, No creo que... is generally more common and natural for I don’t think that... in everyday speech.

Then you would normally switch to the indicative:

• Creo que eso es verdad. = I think that’s true.

This is a very useful pair to remember:

• Creo que... + indicative
• No creo que... + subjunctive

So:

• Creo que eso es verdad.
• No creo que eso sea verdad.

It is neutral and works in both formal and informal situations.

You can say it:

• in everyday conversation
• in polite disagreement
• in writing
• in more formal discussion

If you want to sound softer or more cautious, Spanish often uses similar phrases like:

• No me parece que eso sea verdad.
• No diría que eso sea verdad.

But No creo que eso sea verdad is already perfectly natural and polite.

The key pattern is:

• No creo que + subjunctive

This is one of the most common ways Spanish shows doubt or non-belief.

A good model to memorize is:

• Creo que es...
• No creo que sea...

For example:

• Creo que es posible.
• No creo que sea posible.

If you learn that pattern well, you will be able to build many correct Spanish sentences.