У порога стояли мокрые ботинки, потому что у калитки была большая лужа.

Because both follow the preposition у, which normally requires the genitive.

• у порога = by / near the threshold
• у калитки = by / near the gate

Base forms:

• порог → порога
• калитка → калитки

For English speakers, it helps to think of у as often meaning by, near, or sometimes at the place of.

Both are in the nominative case, because they are the grammatical subjects of their clauses.

• мокрые ботинки = wet boots
• большая лужа = a big puddle

You can see this in the endings:

• мокрые agrees with plural ботинки
• большая agrees with feminine singular лужа

So the sentence structure is basically:

• [At the threshold] stood wet boots
• because [by the gate] there was a big puddle

стоять means to stand, and Russian often uses it for objects that are upright or placed somewhere in a visible position.

So:

• У порога стояли мокрые ботинки literally means Wet boots were standing by the threshold

In natural English, we usually just say There were wet boots by the door / at the doorstep, but Russian often prefers a more concrete verb like:

• стоять = stand
• лежать = lie
• висеть = hang
• сидеть = sit

So стояли sounds very normal here.

Because the verbs agree with different subjects.

First clause

стояли is plural, because the subject is ботинки (boots), which is plural.

Second clause

была is feminine singular, because the subject is лужа (puddle), which is feminine singular.

Past tense in Russian agrees in number, and in the singular also in gender:

• стоял = masculine singular
• стояла = feminine singular
• стояло = neuter singular
• стояли = plural

Likewise:

• был
• была
• было
• были

Russian word order is flexible, and starting with the location often sets the scene first.

So:

• У порога стояли мокрые ботинки puts focus first on where the boots were.
• Мокрые ботинки стояли у порога is also grammatical, but it sounds a bit more neutral or more focused on the boots themselves.

Russian often uses this kind of order to guide attention:

1. location or setting first
2. then the thing/person in that setting

Literally, у порога means by the threshold or near the threshold.

Depending on context, natural English translations could be:

• at the doorstep
• by the door
• near the entrance
• by the threshold

It does not necessarily mean physically on top of the threshold. It usually means in that area.

Калитка is usually a small gate, especially a gate in a fence around a house or garden.

It is not the same as:

• дверь = door
• ворота = large gate / gates

So у калитки была большая лужа suggests a puddle near the yard/garden gate, not near the house door.

Because потому что introduces a subordinate clause meaning because.

Russian normally places a comma before it:

• ..., потому что ...

So the sentence is divided into:

• main clause: У порога стояли мокрые ботинки
• reason clause: потому что у калитки была большая лужа

This is very similar to English punctuation in sentences like:

• The boots were wet, because there was a big puddle by the gate.

Yes, that is grammatically possible, but it feels a bit different.

• У порога стояли мокрые ботинки paints a more visual scene: the boots were standing there
• У порога были мокрые ботинки is more neutral: there were wet boots by the threshold

Russian often prefers a specific positional verb like стоять when talking about objects placed somewhere.

Good question. Russian can express there was in different ways.

Here, the sentence is built like this:

• У калитки была большая лужа
• literally: By the gate was a big puddle

So лужа is the subject, and it stays in the nominative.

Russian also has existential patterns like:

• У меня есть книга = I have a book
• На столе лежала книга = There was a book lying on the table

In your sentence, the location comes first, but лужа is still the grammatical subject.

Because мокрые must agree with ботинки.

Ботинки is:

• plural
• in the nominative case here

So the adjective must also be:

• plural
• nominative

That gives:

• мокрые ботинки = wet boots

Compare:

• мокрый ботинок = wet boot (masculine singular nominative)
• мокрая куртка = wet jacket
• мокрое пальто = wet coat
• мокрые ботинки = wet boots

Yes, sometimes. For example:

• так как = since / because
• из-за того что = because of the fact that

But потому что is the most straightforward and common neutral choice here.

So:

• У порога стояли мокрые ботинки, потому что у калитки была большая лужа.

sounds completely natural.

Using так как would sound a bit more formal or bookish in some contexts.

Yes, it is very natural.

A very literal English version would be:

• By the threshold stood wet boots, because by the gate there was a big puddle.

That sounds marked or literary in English, but it is normal in Russian because Russian allows:

• location-first word order
• specific positional verbs like стоять
• flexible information structure

So even if you would translate it more naturally as:

• There were wet boots by the doorstep because there was a big puddle by the gate,

the Russian original is perfectly ordinary.