Na obálku píšu adresu velmi pomalu, aby tam nebyla chyba.

Because na can take different cases with different meanings:

• na + accusative = movement toward a surface, or putting/writing something onto it
• na + locative = location on a surface

Here, the idea is I am writing the address onto the envelope, so Czech uses na obálku.

Compare:

• Píšu adresu na obálku. = I’m writing the address onto the envelope.
• Adresa je na obálce. = The address is on the envelope.

So na obálku is about the result or placement of the writing.

Adresu is the accusative singular of adresa.

It is in the accusative because it is the direct object of píšu:

• píšu co? → adresu

The basic forms are:

• adresa = nominative
• adresu = accusative

This is very common in Czech: the thing you write, read, see, have, etc. is often in the accusative.

Because píšu is imperfective, and it describes an action in progress.

• píšu = I am writing / I write
• napíšu = I will write / I will finish writing

In this sentence, the speaker is focusing on the process: they are writing very slowly. That is why píšu fits better.

If you said napíšu, it would sound more like completion:

• Napíšu adresu na obálku. = I’ll write the address on the envelope.

But here the idea is I am writing it slowly so there won’t be a mistake, so imperfective píšu is the natural choice.

It means very slowly.

• velmi = very
• pomalu = slowly

So píšu adresu velmi pomalu = I’m writing the address very slowly.

This focuses on speed. The speaker is deliberately slowing down in order to avoid making an error.

A related word is opatrně = carefully/cautiously. That would shift the meaning slightly:

• velmi pomalu = very slowly
• opatrně = carefully

They are close in meaning here, but not identical.

Aby means so that, in order that, or sometimes so.

It introduces a purpose clause: the reason for doing something.

So:

• Píšu adresu velmi pomalu, aby tam nebyla chyba.
• I’m writing the address very slowly so that there won’t be a mistake there.

This is a very common Czech pattern:

• Dělám to, aby... = I’m doing it so that...
• Říkám to, aby... = I’m saying it so that...
• Píšu pomalu, aby... = I’m writing slowly so that...

After aby, Czech uses a special verb form that often looks like a past form, but here it does not mean past time.

Yes, it looks like a past-tense form, but after aby it is not really expressing past time.

In this sentence, aby tam nebyla chyba means:

• so that there is not / will not be a mistake there

The form nebyla agrees with chyba, which is feminine singular.

So:

• chyba = feminine singular
• therefore: byla / nebyla

This is normal after aby. It is better to think of it as part of the purpose construction, not as ordinary past tense.

Because chyba is the subject of nebyla.

Literally, the clause is something like:

• so that there was not a mistake there

In Czech, with být (to be), the thing that is / is not usually appears as the subject:

• Je tam chyba. = There is a mistake there.
• Není tam chyba. = There isn’t a mistake there.
• Aby tam nebyla chyba. = So that there wouldn’t be a mistake there.

So chyba stays in the nominative.

Tam literally means there, but in this sentence it refers to the place where the address is being written — basically on the envelope / in that writing.

So aby tam nebyla chyba means:

• so that there wouldn’t be a mistake there
• more naturally in English: so that there wouldn’t be a mistake in it / in the address

Czech often uses tam in a broad, natural way when the place is clear from context.

You could also say things like:

• aby na obálce nebyla chyba = so that there wouldn’t be a mistake on the envelope
• aby v adrese nebyla chyba = so that there wouldn’t be a mistake in the address

But tam is perfectly natural in everyday speech.

Yes. In standard Czech, a subordinate clause introduced by aby is separated by a comma.

So:

• Na obálku píšu adresu velmi pomalu, aby tam nebyla chyba.

That comma is correct and expected.

Yes. Czech word order is quite flexible, and changing it usually changes emphasis, not the basic meaning.

For example, these are all possible:

• Na obálku píšu adresu velmi pomalu, aby tam nebyla chyba.
• Adresu píšu na obálku velmi pomalu, aby tam nebyla chyba.
• Adresu na obálku píšu velmi pomalu, aby tam nebyla chyba.

The original sentence puts na obálku first, which gives it some emphasis or sets the scene: as for the envelope, I’m writing the address on it...

So the meaning stays basically the same, but the focus shifts.

Yes. Czech has several natural ways to say something very close to this.

For example:

• Na obálku píšu adresu velmi pomalu, abych neudělal chybu.
  = I’m writing the address on the envelope very slowly so that I don’t make a mistake.

• Píšu adresu na obálku velmi pomalu, aby v ní nebyla chyba.
  = I’m writing the address on the envelope very slowly so that there isn’t a mistake in it.

The original sentence is completely fine, but learners should know that Czech often has multiple natural ways to express the same basic idea.

It can be understood either way, depending on context.

Because píšu is imperfective present, it can mean:

• I am writing (right now)
• I write (habitually or as a general approach)

In this sentence, without more context, many people will probably hear it as something happening now:

• I’m writing the address on the envelope very slowly so that there won’t be a mistake.

But Czech present tense often allows both readings. Context tells you which one is meant.