El montón está junto a la puerta.

Because this sentence is talking about location: where the pile is. In Spanish, location is normally expressed with estar, not ser.

• El montón está junto a la puerta = the pile is next to the door.
• Es would usually describe identity or an essential characteristic, not position.

A useful rule for learners is: use estar for where something is.

Here, montón means something like pile, heap, or stack, depending on context.

It refers to a group of things gathered together in a mass. The best English word depends on what the pile is made of:

• un montón de ropa = a pile of clothes
• un montón de arena = a heap of sand
• un montón de libros = a stack/pile of books

So montón is a general word, and the exact translation can vary.

Spanish uses definite articles very often, sometimes more often than English.

• el montón = the pile
• la puerta = the door

Here, both nouns are specific and identifiable, so the definite article is natural.

Also, the article must match the noun’s grammatical gender:

• montón is masculine, so it takes el
• puerta is feminine, so it takes la

Because puerta is a feminine noun in Spanish. Grammatical gender is a property of the noun, and it affects articles and adjectives.

That is why you say:

• la puerta
• una puerta

Many nouns ending in -a are feminine, and puerta follows that common pattern. But remember that grammatical gender does not always have anything to do with biological sex.

Junto a means next to, beside, or right by.

It is very close in meaning to al lado de, and in many situations they are interchangeable:

• El montón está junto a la puerta
• El montón está al lado de la puerta

Both mean the pile is beside the door.

A small nuance:

• junto a can sound a little more compact or slightly more formal in some contexts
• al lado de is extremely common and very natural in everyday speech

Because junto normally goes with the preposition a when it means next to.

So the standard pattern is:

• junto a + noun

Examples:

• junto a la ventana
• junto al coche
• junto a mí

Using junto de here would not be the normal choice for standard Spanish in this meaning.

There is no contraction because the article is la, not el.

In Spanish, contraction happens only with:

• a + el = al
• de + el = del

So:

• junto a la puerta stays a la
• but junto al edificio would be a + el = al

So if the noun were masculine, you might get a contraction:

• junto al armario

The accent marks show which syllable is stressed.

• montón is stressed on the last syllable: mon-TÓN
• está is stressed on the last syllable: es-TÁ

The written accents are important because they show pronunciation and can sometimes distinguish forms.

For example:

• esta = this
• está = is

So the accent in está is especially important.

A simple approximate guide for Spain Spanish:

• junto ≈ HOON-toh
  The j sounds like a strong throaty sound, not like English j in jump.

• montón ≈ mon-TON
  Stress on the last syllable.

• puerta ≈ PWER-ta
  The ue forms a diphthong, so it sounds like pwer.

Also:

• the r in puerta is a tapped Spanish r, lighter than an English r
• the t and d sounds in Spanish are usually cleaner and less aspirated than in English

Yes. Spanish word order is flexible, although El montón está junto a la puerta is the most neutral and straightforward version.

You could also say:

• Junto a la puerta está el montón.

This version puts more focus on the location first. It can sound a bit more literary or emphatic, depending on context.

For most learners, the original order is the safest and most natural default:

• subject + verb + location

Yes, in some contexts. A common alternative is:

• El montón se encuentra junto a la puerta.

This also means the pile is next to the door. However:

• está is simpler and more common in everyday speech
• se encuentra can sound a bit more formal, descriptive, or written

So for normal conversation, está is usually the best choice.

Yes. Besides meaning pile or heap, montón is also very commonly used in the expression un montón de, meaning a lot of or loads of.

For example:

• Tengo un montón de trabajo = I have a lot of work
• Hay un montón de gente = There are loads of people

That is a different use from the one in your sentence, but it is extremely common, so it is worth learning early.