Nasa plato ang patay na isda.

Because Filipino very often puts the predicate first.

In this sentence:

Nasa plato = the predicate, telling the location
ang patay na isda = the topic, the thing being talked about

So the structure is closer to:

On the plate is the dead fish.

In natural English, we usually switch that to The dead fish is on the plate.

There is no separate word for is here.

In Filipino, especially in simple present-time sentences about identity or location, a separate copula like English is is often not needed.

So:

Nasa plato ang patay na isda
literally works like
On the plate the dead fish

but naturally means
The dead fish is on the plate.

Nasa means something like is in / is at / is on depending on context.

It is commonly explained as a contraction of na sa.

Here:

nasa plato = on the plate

In other contexts, nasa could also mean:

• in the box
• at home
• on the table

So the exact English preposition depends on the situation.

Because sa is a general location marker in Filipino. It does not divide space as narrowly as English prepositions do.

English forces you to choose between in, on, and at. Filipino often just uses sa or nasa, and the listener understands the exact relationship from context.

Since a fish is resting on a plate, English naturally says on the plate.

Ang is a marker, not simply the English word the.

It marks the noun phrase that is the topic or focused noun phrase of the sentence:

ang patay na isda

So ang tells you that the dead fish is the main thing being talked about.

Sometimes ang may line up with English the, but they are not the same kind of word.
It is better to think of ang as a grammatical marker, not a direct translation.

Because na is a linker. It connects the modifier patay to the noun isda.

So:

patay = dead
isda = fish
patay na isda = dead fish

The linker is very common in Filipino when one word modifies another.

Because the form of the linker depends on the word before it.

Here, the modifier is patay, which ends in y, so the normal form is na:

patay na isda

By contrast, when the first word ends in a vowel, you often see -ng attached instead.

Example:

maganda + bahay → magandang bahay

So this is just a linker pattern you gradually get used to.

Yes. That is also correct.

Compare:

Nasa plato ang patay na isda.
Ang patay na isda ay nasa plato.

They mean essentially the same thing.

The second version uses ay because the topic phrase is placed first. This is often called ay-inversion.

A useful point: ay is not the same as English is.
It is mainly a marker used in that sentence pattern.

No. In this sentence, patay is an adjective meaning dead.

The verb to die is different, for example:

mamatay = to die
namatay = died

So:

patay na isda = dead fish
not
fish that dies

By itself, isda can be understood as fish in a general or singular sense, depending on context.

If you want to make the plural explicit, you usually add mga:

ang mga patay na isda = the dead fish / the dead fishes

So:

Nasa plato ang mga patay na isda.
= The dead fish are on the plate.

Because Filipino does not use articles the same way English does.

sa plato can mean:

• on a plate
• on the plate

The exact meaning depends on context.

So in this sentence, English naturally chooses the plate, but Filipino does not need a separate article there.