У двери высокий порог.

Because у is a preposition meaning by, near, at in this kind of sentence. After у, Russian normally uses the genitive case.

So:

• дверь = the door
• у двери = by the door / at the door

This is a very common pattern in Russian:

• у окна = by the window
• у дома = by the house
• у входа = by the entrance

So у двери is not the subject. It is a location phrase.

Because дверь is in the genitive singular after the preposition у.

The noun дверь is feminine and belongs to the soft-sign pattern. Its forms include:

• nominative: дверь
• genitive: двери
• dative: двери
• accusative: дверь
• instrumental: дверью
• prepositional: двери

So here:

• у двери = at/by the door

Even though двери can also be other cases, after у it is understood as genitive.

Yes, from an English point of view it feels like a verb is missing, but in Russian the verb to be is usually not expressed in the present tense.

So:

• У двери высокий порог. literally looks like By the door high threshold.
• Natural English: There is a high threshold at the door or The door has a high threshold, depending on context.

Russian often leaves out есть in simple present-tense statements:

• Он студент. = He is a student.
• Дом большой. = The house is big.

You may sometimes see есть, but not in this basic neutral sentence.

The grammatical subject is порог.

Why?

Because порог is in the nominative case, and it is the thing being described as высокий.

Structure:

• У двери = location phrase (at/by the door)
• высокий порог = a high threshold / the threshold is high

So even though English might begin with there is, Russian simply places the noun phrase in the sentence and lets the context do the work.

Because порог is a masculine singular noun, and adjectives must agree with the noun they describe in:

• gender
• number
• case

Here:

• порог = masculine singular nominative
• so the adjective must also be masculine singular nominative: высокий

Compare:

• высокий порог = a high threshold
• высокая дверь = a tall/high door
• высокое окно = a tall/high window

Порог means threshold, doorstep, or the raised strip at the bottom of a doorway.

It is a masculine noun:

• порог = threshold

In this sentence, it refers to the fact that stepping through the doorway is difficult or noticeable because the threshold is high.

Be careful not to confuse it with other kinds of “limits” or “thresholds” in abstract meanings. Russian also uses порог metaphorically, but here it is the physical doorway threshold.

Russian word order is flexible. Starting with У двери puts the location first, which is very natural if the speaker wants to set the scene first.

So:

• У двери высокий порог. = At the door, there is a high threshold.
• Высокий порог у двери. = The threshold at the door is high.

Both are possible, but they do not feel exactly the same in emphasis.

A rough idea:

• У двери высокий порог → first tells you where
• Высокий порог у двери → first tells you what kind of threshold

Russian often uses word order to guide attention rather than to mark grammar.

Yes, you could, but у двери is the most natural and compact choice here.

These all can mean something like by/near the door:

• у двери
• около двери
• возле двери

Differences:

• у двери often feels very natural for something right at the door or doorway
• около двери and возле двери emphasize physical nearness a bit more explicitly

For a threshold specifically, у двери sounds especially idiomatic because a threshold belongs right at the doorway.

Russian has no articles, so the sentence itself does not directly mark a vs the.

So высокий порог could be understood as:

• a high threshold
• the threshold is high
• the door has a high threshold

The exact English choice depends on context.

This is very common in Russian: you must infer definiteness from the situation, not from an article.

A standard pronunciation would be:

У две́ри высо́кий поро́г.

Stress:

• две́ри
• высо́кий
• поро́г

The preposition у is unstressed and attaches closely to the following word.

So it sounds roughly like:

u DVYE-ri vi-SO-kee pa-ROK

Not here. The preposition у can be used in different ways.

1. Possession

   • У меня есть книга. = I have a book.
   • literally: At me there is a book.

2. Location / nearness

   • У двери стоит стул. = A chair is standing by the door.
   • У двери высокий порог. = There is a high threshold at the door.

So in your sentence, у двери is a location phrase, not a possessor.

Yes. Even though the structure is literally more like At the door there is a high threshold, English often expresses this idea as The door has a high threshold.

That is a normal interpretation, especially in practical contexts, for example when warning someone about accessibility or stepping carefully.

So depending on context, natural English could be:

• There is a high threshold at the door.
• The door has a high threshold.
• The threshold by the door is high.

Russian does not force one single English rendering here.