Ada genangan air hujan di dekat tangga.

In this sentence, ada means there is / there are. It introduces the existence of something.

So:

• Ada genangan air hujan... = There is a puddle of rainwater...

A very common pattern in Indonesian is:

• Ada + noun + location
• There is/are + noun + location

For example:

• Ada mobil di luar. = There is a car outside.
• Ada orang di dapur. = There is someone in the kitchen.

Note that ada does not mean have in this sentence.

Genangan means something like a puddle, a pool of standing liquid, or a patch of water that has collected.

It comes from the root genang, which has to do with water collecting or pooling.
So genangan air hujan is literally a pooling of rainwater or more naturally a puddle of rainwater.

This word is useful when the water is not flowing, but sitting in one place.

This is one of the most common beginner confusions.

In Indonesian:

• air = water
• udara = air (the stuff we breathe)

So in this sentence:

• air hujan = rainwater

not rain air in the English sense.

Examples:

• air minum = drinking water
• air panas = hot water
• air laut = seawater

Air hujan means rainwater.

This is a very normal Indonesian noun phrase:

• air = water
• hujan = rain

Together:

• air hujan = rainwater

So genangan air hujan means a puddle of rainwater.

Indonesian often builds noun phrases by putting the main noun first and the modifier after it.

Because Indonesian usually puts the main noun first, followed by words that describe or specify it.

Here the main thing is:

• genangan = puddle / pooling

What kind of puddle?

• air hujan = rainwater

So:

• genangan air hujan = puddle of rainwater

This is different from English, where we often use of:

• puddle of rainwater

Indonesian usually does not need a word equivalent to of in this structure.

Di dekat means near or close to.

So:

• di dekat tangga = near the stairs / near the staircase

A few similar location expressions are:

• di atas = on / above
• di bawah = under / below
• di samping = beside
• di depan = in front of
• di belakang = behind
• di dekat = near

Because here di is a preposition meaning in / at / on, and prepositions are written separately.

So:

• di dekat = near
• di rumah = at home
• di meja = on the table

This is different from the prefix di- used in passive verbs, which is written together:

• ditulis = written
• dibuka = opened

So a useful rule is:

• di + place/location → separate
• di- + verb → together

Tangga can mean stairs, staircase, or sometimes ladder, depending on context.

In this sentence, di dekat tangga will most naturally mean:

• near the stairs
• near the staircase

If the context were construction or tools, it might mean ladder, but in everyday situations people will usually understand stairs/staircase.

Indonesian does not usually use articles like a, an, or the.

So:

• tangga can mean stairs, the stairs, or a staircase
• genangan air hujan can mean a puddle of rainwater or the puddle of rainwater, depending on context

Context tells you whether something is specific or not.

That is why one Indonesian sentence can often be translated in more than one natural English way.

By default, it sounds singular: there is a puddle of rainwater near the stairs.

However, Indonesian nouns usually do not change form for singular vs. plural.
So without extra context, genangan could sometimes be understood more generally.

If someone clearly wanted to emphasize plural puddles, they might say something like:

• Ada beberapa genangan air hujan di dekat tangga.
  = There are several puddles of rainwater near the stairs.

So number is often shown by context or added words like:

• beberapa = several
• banyak = many

You can, but it changes the feel.

• Ada genangan air hujan di dekat tangga.
  = a full, normal sentence: There is a puddle of rainwater near the stairs.

• Genangan air hujan di dekat tangga.
  = more like a note, label, warning, headline, or sentence fragment

So in ordinary speech, ada is the more complete and natural choice if you are simply stating that something is there.

Yes, it is natural and correct.

It sounds like a normal descriptive sentence, especially if someone is pointing out a hazard or explaining a situation.

A few close alternatives are also possible, depending on what you want to emphasize:

• Ada genangan air di dekat tangga.
  = There is a puddle near the stairs.
  (You do not specify that it is rainwater.)

• Ada air hujan yang menggenang di dekat tangga.
  = There is rainwater pooled near the stairs.
  (This emphasizes the water collecting there.)

But the original sentence is perfectly good Indonesian.

It would often be used to point out something physically present, especially a possible safety issue.

For example:

• warning someone not to slip
• explaining why the floor is wet
• describing what you see near an entrance or staircase

Because genangan air hujan suggests standing water, the sentence can imply:

• Be careful
• It might be slippery
• Rainwater has collected there

So even if the sentence is just descriptive, the context may carry a practical warning.