Pod naszym blokiem jest podziemny parking, więc samochód stoi tam bezpiecznie.

Under our apartment building there is an underground parking lot, so the car stands there safely.

Breakdown of Pod naszym blokiem jest podziemny parking, więc samochód stoi tam bezpiecznie.

• być: to be
• samochód: the car
• więc: so
• nasz: our

Questions & Answers about Pod naszym blokiem jest podziemny parking, więc samochód stoi tam bezpiecznie.

What does blok mean here? Is it the same as English block?

In this context blok means an apartment building, usually a multi-storey residential building (often concrete, but not necessarily).

It does not mean:

• a city block of streets, or
• a physical block (piece of wood, etc.).

So pod naszym blokiem = under our apartment building / under our block of flats.

Why is it pod naszym blokiem and not pod nasz blok or pod naszym blok?

The preposition pod can take two different cases, depending on the meaning:

• Location (where?) → instrumental
  
  • pod naszym blokiem = under our block (and it stays there)
• Movement (to where?) → accusative
  
  • Idę pod nasz blok. = I’m going to (the area) under our block.

In the sentence you gave, we are talking about a location (the parking already exists under the building), not movement, so instrumental is used: naszym blokiem.

pod naszym blok is simply ungrammatical: after pod in the static sense, you must change the endings to the instrumental form.

What case is naszym blokiem, and how are those endings formed?