Na obálce ještě není známka ani adresa.

Because na can take different cases depending on meaning:

• na + accusative = movement onto something
• na + locative = location on something

Here nothing is moving. The stamp and address are being described as located on the envelope, so Czech uses the locative:

• obálka → na obálce

So na obálce means on the envelope.

Because the sentence is talking about things written or attached on the outside surface of the envelope.

• na obálce = on the envelope
• v obálce = in the envelope

A stamp and an address normally go on an envelope, not in it, so na is the natural choice.

In this sentence, ještě means yet.

So ještě není is understood as is not yet.

Even though ještě often literally means still, in negative sentences it very often corresponds to English yet:

• Ještě není doma. = He isn't home yet.
• Na obálce ještě není známka ani adresa. = There isn't a stamp or address on the envelope yet.

Czech does not need a dummy subject like English there.

English says:

• There is no stamp on the envelope.

Czech simply uses the verb být (to be) without any extra placeholder:

• Na obálce není známka.

So literally it is more like:

• On the envelope is not a stamp.

That is normal Czech structure.

Because in this kind of existential sentence, Czech often uses singular when the list comes after the verb and the sentence presents the absence of items as one situation.

So:

• Na obálce ještě není známka ani adresa.

is a normal, idiomatic way to say it.

If the nouns were moved forward and treated more clearly as the grammatical subject, plural would be more likely:

• Známka ani adresa na obálce ještě nejsou.

So the singular here is not strange; it fits the sentence structure and focus.

Here ani means something like nor or or in a negative sentence.

So:

• známka ani adresa

means:

• a stamp nor an address
• more naturally in English: a stamp or an address
• or: neither a stamp nor an address

In Czech, after a negative verb, ani is the normal connector for this kind of meaning.

Both patterns are possible, but they are slightly different in style.

The sentence you have:

• není známka ani adresa

is a natural, compact way to say there is neither a stamp nor an address.

A more emphatic version would be:

• není ani známka, ani adresa

That repeats ani and sounds stronger. So one ani is enough here; repeating it just adds emphasis.

Because they are in the nominative singular.

With být in this kind of sentence, the things that are or are not present are normally named in the nominative:

• je tam známka
• není tam známka

So:

• známka
• adresa

stay in their basic forms.

Czech has no articles like English a/an/the.

So bare nouns like známka and adresa can correspond to different English choices depending on context. In this sentence, English naturally uses the indefinite meaning:

• a stamp
• an address

because the sentence is talking about whether such things are present on the envelope, not about specific already-known ones.

Czech word order is fairly flexible, but changing it changes the emphasis.

The given order:

• Na obálce ještě není známka ani adresa.

is natural and neutral.

Other orders are possible, for example:

• Ještě na obálce není známka ani adresa.
• Známka ani adresa na obálce ještě nejsou.

These are not identical in focus. Czech often puts old/background information earlier and the most important new information later. Here, the missing items známka ani adresa come near the end, which sounds very natural.

Yes. In this context, známka means postage stamp.

The word známka can mean other things in other contexts, such as:

• a school grade
• a mark
• a sign

But with obálka (envelope), the meaning is clearly stamp. If you want to be extra explicit, you can say poštovní známka, but Czech often just says známka because the context makes it obvious.