En el festival hay un puesto donde venden palomitas dulces y otro donde sirven agua fría y zumo.

Hay means there is / there are and is used to say that something exists or is present somewhere.

So:

• En el festival hay un puesto... = At the festival, there is a stall...

You would use está / están more for saying where a specific thing is located:

• El puesto está en el festival. = The stall is at the festival.

So in this sentence, the speaker is introducing the existence of the stall, which is why hay is the natural choice.

Here, puesto means a stall, stand, or booth, like a small place where food or drinks are sold at a festival, market, or fair.

Common related words:

• un puesto de comida = a food stall
• un puesto de helados = an ice cream stand
• un puesto del mercado = a market stall

In this context, puesto is a very natural word for a temporary place selling things at an event.

Because donde here is a relative word, not a direct or indirect question.

• donde = where, linking to a noun already mentioned
• dónde = where?, in a question or exclamation

So:

• un puesto donde venden palomitas = a stall where they sell popcorn
• ¿Dónde venden palomitas? = Where do they sell popcorn?

The accent mark appears in interrogative or exclamatory uses, but not in this kind of descriptive clause.

This is a very common Spanish structure. The verbs are in the third person plural to mean something like:

• they sell
• they serve
• people sell
• people serve

It often refers to unspecified people working there.

So:

• donde venden palomitas dulces = where they sell sweet popcorn
• donde sirven agua fría y zumo = where they serve cold water and juice

English does this too with they, but Spanish uses it very naturally in this kind of context.

Yes, that is possible, but it changes the style slightly.

For example:

• un puesto donde venden palomitas dulces
• un puesto donde se venden palomitas dulces

Both can mean roughly a stall where sweet popcorn is sold.

The version with venden / sirven sounds more like people there sell/serve it.
The version with se venden / se sirve is more impersonal and focuses less on the workers.

In everyday speech, the original version is very natural.

In Spanish, adjectives usually come after the noun.

So:

• palomitas dulces = sweet popcorn
• agua fría = cold water

The adjective dulces has to agree in number with palomitas, which is plural:

• palomita dulce = one sweet popcorn piece / one popcorn kernel or unit in theory
• palomitas dulces = sweet popcorn

Even though English often puts adjectives before nouns, Spanish usually places them after.

Because it describes palomitas, which is plural.

Agreement in Spanish means adjectives match the noun in:

• gender
• number

Here:

• palomitas = feminine plural
• dulces = plural form of dulce

So the adjective must also be plural.

Compare:

• una bebida fría
• dos bebidas frías

Because otro is standing in for otro puesto. Spanish often omits a noun when it is already understood from the context.

So:

• hay un puesto... y otro...

really means:

• hay un puesto... y otro puesto...

This is very natural and avoids repetition. English does the same:

• There is one stall... and another...

Because it refers back to puesto, which is a masculine noun:

• el puesto
• otro puesto

If the noun were feminine, you would use otra:

• una tienda... y otra...

So otro agrees with the implied noun puesto.

Because Spanish often leaves out the article when talking about things in a general, non-specific way, especially after verbs like servir, vender, comer, beber, etc.

So:

• sirven agua fría y zumo = they serve cold water and juice

This sounds like the kinds of things available there, not specific items already identified.

You could add articles in some contexts, but it would change the nuance:

• sirven el agua fría y el zumo would sound like specific water and specific juice already known in the conversation.

So the version without articles is the most natural here.

Good question. Agua is a feminine noun, but in the singular it often takes el before it:

• el agua fría

This happens because agua begins with a stressed a sound, and Spanish avoids la agua.

However, in your sentence there is no article at all:

• sirven agua fría

So the issue is not that the article changed; it is that the article is simply omitted.

Also remember:

• el agua fría → article is masculine in form
• fría stays feminine because agua is still a feminine noun

Yes. In Spain, zumo is the usual word for juice.

In much of Latin America, people often say jugo instead.

So:

• Spain: zumo de naranja
• many Latin American countries: jugo de naranja

Since you are learning Spanish from Spain, zumo is the expected word here.

Yes. Here servir means to serve food or drinks.

So:

• sirven agua fría y zumo = they serve cold water and juice

Be aware that servir can have other meanings too, such as:

• to be useful
• to work/function
• to be suitable

Examples:

• Esto no sirve. = This is no good / This doesn’t work.
• Me sirve esta mesa. = This table works for me.

But in a food stall context, servir very naturally means to serve.

Both are possible.

• En el festival hay un puesto...
• Hay un puesto... en el festival

The original version puts En el festival first to set the scene: At the festival...

This word order is very common when the speaker wants to establish the location before saying what is there.

So it is mostly a matter of emphasis and information flow, not a major grammar difference.

This is a noun followed by a relative clause.

• un puesto = a stall
• donde venden palomitas dulces = where they sell sweet popcorn

So donde introduces extra information about puesto.

You can think of it as:

• a stall where they sell sweet popcorn

This is a very useful pattern in Spanish:

• una tienda donde venden ropa = a shop where they sell clothes
• un bar donde sirven tapas = a bar where they serve tapas

Yes. Palomitas is the normal word for popcorn in Spain.

Other regions may use different words, for example:

• palomitas = common in Spain
• pochoclo, pororó, cotufas, crispetas, etc. = used in different Latin American countries

So for Spanish from Spain, palomitas is exactly the right word.

Because the sentence is joining two parallel parts with y:

• un puesto donde venden palomitas dulces
• otro donde sirven agua fría y zumo

In Spanish, you normally do not put a comma before y in a simple coordination like this.

So the punctuation here is standard and natural.