Learn Hardcore Czech: Nevím, co je pravda. - I don't know what is true.

The verb is vědět = “to know (a fact)”.
vím = “I know” (1st person singular, present).
The negative prefix ne- is added in front of the verb: ne + vím → nevím = “I don’t know”.

So Nevím, co je pravda. is literally “I-don’t-know what is truth.”

Why is there a comma before co?

Czech always separates a subordinate clause from the main clause with a comma.

Nevím is the main clause: “I don’t know”.
co je pravda is a subordinate clause (an indirect question): “what is true / what the truth is”.

Therefore you must write: Nevím, co je pravda.
Leaving out the comma is incorrect in standard written Czech.

What exactly does co mean here, and why is it used?

Co is the basic interrogative pronoun “what”.

In Nevím, co je pravda, it introduces an indirect question:

Direct question: Co je pravda? – “What is the truth?”
Indirect: Nevím, co je pravda. – “I don’t know what the truth is.”

So co works like English “what” in “I don’t know what …”.

Grammatically:

In the clause co je pravda, both co and pravda are in the nominative case, because with “to be” both sides usually appear in the nominative.
Here, co is the “unknown thing” (the question word) and pravda is the complement: “this unknown thing is ‘truth’”.

Why is the word order co je pravda and not co pravda je?

Czech word order is relatively flexible, but there is a neutral order that sounds the most natural.

Co je pravda is the neutral, most common word order in both the direct question (Co je pravda?) and the indirect one (Nevím, co je pravda.).
Co pravda je is possible, but it sounds less neutral and more marked, as if you were putting extra emphasis on pravda (“what truth actually is”).

For a learner, it’s best to treat co je pravda as the normal pattern here.

Why does the Czech indirect question keep the same word order as the direct question, unlike English?

In English:

Direct: What is the truth?
Indirect: I don’t know what the truth is. (you move is to the end)

In Czech, the word order in a subordinate “question clause” usually stays the same as in the direct question:

Direct: Co je pravda?
Indirect: Nevím, co je pravda.

So you do not move je to the end the way English moves is. You simply embed the question as it stands, and change only the punctuation and intonation (it becomes a statement overall, so no question mark).

Why is there no word for “the” before pravda?

Czech has no articles (a, an, the). Whether English would use “a truth” or “the truth” is inferred from context.

pravda on its own can mean “truth”, “the truth”, or sometimes “a truth” depending on context.
If you really want to emphasize something like “that particular truth”, you might add a demonstrative:
- Nevím, jaká je ta pravda. – “I don’t know what that truth is.”

But in everyday language, Nevím, co je pravda. naturally corresponds to “I don’t know what the truth is / what is true.”

Could I say Nevím, jaká je pravda instead? Is there a difference?

Yes, you can say Nevím, jaká je pravda.

Nuance:

Nevím, co je pravda.
- Focuses on which statements / which version is true.
- Very close to “I don’t know what is true / what the truth is.”
Nevím, jaká je pravda.
- Literally “I don’t know what kind (what sort) the truth is.”
- Often feels a bit more descriptive or philosophical: you don’t know the nature or character of the truth, how it really is.

In many contexts they can overlap, but co je pravda sounds slightly more like choosing between competing claims; jaká je pravda sounds slightly more like not knowing the nature or full picture of the truth.

Could I drop je and say Nevím, co pravda?

No. You need the verb je (“is”) here.

In Czech, the verb být (“to be”) is usually expressed in the present tense in such sentences:

Correct: Nevím, co je pravda. – “I don’t know what the truth is.”
Nevím, co pravda. – ungrammatical / incomplete.

There are some patterns where je can be omitted (especially in short colloquial answers or headlines), but this kind of full sentence is not one of them.

Why is the sentence ending with a period and not a question mark, even though it contains a question word?

The main clause Nevím (“I don’t know”) is a statement, not a direct question. The part with co is an indirect question embedded inside that statement.

Compare:

Direct question: Co je pravda? – ends with a question mark.
Statement about a question: Nevím, co je pravda. – ends with a period, because overall you are stating that you don’t know.

So you only use a question mark when the entire sentence is being asked as a question.

What does pravda mean, and can it also mean “true”?

pravda is a noun meaning “truth”.

Pravda (noun) = “truth”.
pravdivý (adjective) = “true, truthful”.
pravda by itself does not mean “true” as an adjective.

Examples:

To je pravda. – “That’s true.” (literally “That is truth.”)
Nevím, co je pravda. – “I don’t know what is true / what the truth is.”
To je pravdivý příběh. – “That is a true story.”

So in your sentence, pravda is the noun “truth,” not an adjective.

How would I say this in the past tense: “I didn’t know what the truth was”?

You keep co je pravda in the present if you mean a general or still-valid truth, but change the main verb nevím to the past:

Nevěděl jsem, co je pravda. – “I didn’t know what the truth was.” (said by a man)
Nevěděla jsem, co je pravda. – same (said by a woman)

If you really want to highlight that even the truth itself is placed in the past (less common), you can say:

Nevěděl jsem, co byla pravda.
Nevěděla jsem, co byla pravda.

But for the usual sense (“back then, I didn’t know what (is) true”), Nevěděl(a) jsem, co je pravda is the natural choice.