У порога стоит старый стул, а за калиткой начинается тихий двор.

to stand

Breakdown of У порога стоит старый стул, а за калиткой начинается тихий двор.

• стул: the chair
• тихий: quiet
• у: by

Questions & Answers about У порога стоит старый стул, а за калиткой начинается тихий двор.

Why is it у порога? What case is порога?

У means by, near, at, and it normally requires the genitive case.

So:

• порог = threshold, doorstep
• у порога = by the threshold / at the doorstep

That is why порог changes to порога.

This is a very common pattern:

• у дома = by the house
• у двери = by the door
• у окна = by the window

So у порога стоит старый стул literally has the structure By the threshold stands an old chair.

Why is it за калиткой, not за калитку?

Because за can take different cases depending on the meaning.

• за + instrumental = location, behind/beyond
• за + accusative = motion, to behind / going beyond

Here there is no movement behind the gate. It describes where something begins from the speaker’s point of view: behind/beyond the gate. So Russian uses the instrumental:

• калитка → калиткой

Compare:

• за калиткой начинается тихий двор = beyond the gate, a quiet yard begins
• идти за калитку = to go beyond the gate

So за калиткой is a location phrase.