Zjistím, zda je to pravda.

Because zjistím is from the perfective verb zjistit.

In Czech, perfective verbs do not normally have a true present meaning. Their present-tense forms usually refer to the future. So:

• zjistím = I will find out
• not usually I find out in the general present sense

If you wanted an ongoing or repeated present meaning, Czech would normally use the imperfective partner, such as zjišťovat.

The infinitive is zjistit.

This is the perfective verb meaning to find out, to ascertain, or to discover.

A useful aspect pair is:

• zjistit = perfective, to find out / to discover
• zjišťovat = imperfective, to be finding out / to check / to investigate

So zjistím is:

• 1st person singular
• from zjistit
• meaning I will find out

Zda means whether.

It introduces an indirect yes/no question, just like English whether:

• Zjistím, zda je to pravda.
• I’ll find out whether it’s true.

It is more formal or neutral in style than jestli, which is also very common.

Yes. Zjistím, jestli je to pravda. is completely natural.

The difference is mainly one of style:

• zda = more formal, written, or slightly more careful
• jestli = very common in everyday speech

Both mean whether here.

So these are both fine:

• Zjistím, zda je to pravda.
• Zjistím, jestli je to pravda.

Because zda je to pravda is a subordinate clause.

Czech normally separates subordinate clauses with a comma, so:

• Zjistím, zda je to pravda.

This is standard Czech punctuation.
The comma is required before conjunctions like zda, jestli, že, protože, and so on when they introduce a subordinate clause.

Literally, it is something like:

• je = is
• to = it / that
• pravda = truth

So word-for-word, it is roughly it is truth.

But natural English is:

• it is true
• or that is true

Czech often uses pravda (truth) in places where English prefers the adjective true.

Because after být (to be), Czech normally uses the nominative for the predicate noun.

So:

• To je pravda. = That is true.
• Je to pravda. = It is true.

Here, pravda is not a direct object. It is the noun that completes the meaning of je (is), so it stays in the nominative form.

That is why pravda is correct, not pravdu.

To refers to the thing being discussed: it, that, or the statement in question.

So:

• je to pravda = is it true / it is true

In this sentence, to is very natural and normally expected. It points to whatever claim or situation the speaker wants to check.

Without to, je pravda would usually sound incomplete in this context.

Yes, that is possible.

Both of these are understandable:

• Zjistím, zda je to pravda.
• Zjistím, zda to je pravda.

The first version, zda je to pravda, is the more neutral and standard phrasing.

Czech word order is flexible, but changes in order can slightly change emphasis or style. A learner should probably use:

• Zjistím, zda je to pravda.

as the safest default.

It sounds neutral to slightly formal, mainly because of zda.

• Zjistím, zda je to pravda. = neutral/written/slightly formal
• Zjistím, jestli je to pravda. = more everyday and conversational

The rest of the sentence is completely normal in all kinds of Czech.

A very common everyday version would be:

• Zjistím, jestli je to pravda.

That is probably what you would hear more often in conversation.

The version with zda is still correct and natural, but it feels a bit more formal or written.

Approximately: zjis-teem

A few details:

• zj is a consonant cluster, so the word starts with a compact zj sound
• stí has a long í, so the final syllable is longer
• stress in Czech is almost always on the first syllable, so: ZJIS-tím

You do not need to exaggerate the stress, but the first syllable should be the stressed one.