El puré de patata está tan suave que mi sobrina quiere más.

Because estar is usually used for a state or condition, while ser is used for something more essential or defining.

Here, suave describes the mashed potato’s current texture: it has turned out soft/smooth. That is a condition, so está sounds natural.

• El puré está suave = the mashed potato is soft/smooth right now.
• Es suave would sound more like softness is a general characteristic of that kind of thing, which is less natural here.

So in food contexts, texture at the moment is very often expressed with estar.

Tan ... que means so ... that.

It is a very common structure in Spanish:

• tan
  • adjective/adverb + que
• so
  • adjective/adverb + that

In your sentence:

• tan suave = so smooth/soft
• que mi sobrina quiere más = that my niece wants more

Other examples:

• Está tan caliente que no puedo comerlo. = It’s so hot that I can’t eat it.
• Habla tan rápido que no entiendo nada. = He speaks so fast that I don’t understand anything.

A common learner mistake is using muy here. If you want so ... that, you need tan ... que, not muy.

In Spain, both puré de patata and puré de patatas can be heard, but puré de patata is very common and natural.

The singular patata is often used after de when naming a dish or ingredient in a general way, similar to saying:

• tortilla de patata
• crema de calabaza
• zumo de naranja

It does not mean there is only one potato. It just names the type of purée: potato purée / mashed potato.

So:

• puré de patata = very normal in Spain
• puré de patatas = also possible

The noun puré is masculine, so it takes el:

• el puré

Unfortunately, noun gender is not always predictable from meaning alone, so you often have to learn it with the article.

A useful habit is to memorize nouns as a unit:

• el puré
• la patata
• la sobrina

Also, nouns ending in -é are often masculine, though not always. The safest approach is simply to learn el puré as a fixed combination.

In this sentence, suave refers to the texture of the food. It can mean:

• smooth
• soft
• sometimes even creamy, depending on context

With mashed potato, smooth is often the best match. It suggests there are no lumps and the texture is pleasant.

Be aware that suave has a wider meaning in Spanish than soft in English. It can also describe:

• a mild flavour
• a gentle touch
• a smooth surface
• a soft sound

So here the exact English word depends on context, but for food texture smooth/soft fits well.

Más has a written accent because it means more.

• más = more
• mas = but

The form mas without an accent exists, but it is literary or old-fashioned in modern Spanish. In everyday Spanish, but is normally pero.

So in your sentence:

• quiere más = wants more

The accent helps distinguish it from the different word mas.

Because Spanish, like English, can omit a noun when it is obvious from context.

Here, everyone knows the topic is el puré de patata, so más on its own means more of it / more mashed potato.

So these are both natural:

• Mi sobrina quiere más.
• Mi sobrina quiere más puré.

The shorter version is very common because the noun is already understood.

Yes, Spanish often drops subject pronouns, but here mi sobrina is not a pronoun: it is the full subject noun phrase.

The sentence could be reduced if the context already made the subject clear, but including mi sobrina is perfectly normal because it tells you who wants more.

Compare:

• Mi sobrina quiere más. = My niece wants more.
• Quiere más. = She wants more. / He wants more. / You want more.
  This would depend on context.

Spanish frequently omits pronouns like yo, tú, él, ella, but it still uses full nouns like mi sobrina whenever needed for clarity or emphasis.

In modern standard Spanish, possessive adjectives like mi, tu, su, nuestro usually go directly before the noun with no article:

• mi sobrina
• tu casa
• su libro

So mi sobrina simply means my niece.

Using an article with a possessive in this position is not standard modern Spanish. There are some special or old-fashioned constructions in certain varieties or literary styles, but for normal Spanish from Spain, mi sobrina is exactly right.

Yes, but it would change the structure and slightly change the meaning.

• Está muy suave. = It is very smooth/soft.
• Está tan suave que mi sobrina quiere más. = It is so smooth/soft that my niece wants more.

So:

• muy = very
• tan ... que = so ... that

If you want to express a consequence (that my niece wants more), then tan ... que is the correct pattern.

Yes, it sounds especially natural in Spain, because patata is the usual word there.

In much of Latin America, people often say papa instead of patata. So an equivalent sentence in many Latin American varieties might use:

• puré de papa

Since you asked about Spanish from Spain, puré de patata is exactly the kind of phrasing learners should expect to hear there.