Learn Hardcore Russian: Сейчас большой зал пустой.

Adding есть here would sound either wrong or at least very unnatural. The meaning “Now the big hall is empty” is understood without any explicit is.

Could I ever say есть in this kind of sentence?

Yes, but with a different structure and meaning.

есть is used most often:

To say that something exists or is available:
- У нас есть большой зал. – We have a big hall / There is a big hall.
For emphasis in some contexts (more literary or emotional):
- Он есть герой. – He really is a hero. (unusual in everyday speech)

But in Сейчас большой зал пустой, you should not insert есть; it does not fit the normal pattern for “X is Y” in the present tense.

Why do большой and пустой both end in -ой? Is that just coincidence?

Not a coincidence. Both are adjectives in the same form:

большой – masculine, singular, nominative (long form)
пустой – masculine, singular, nominative (long form)

They agree with зал, which is:

зал – masculine, singular, nominative

So all three match in gender (masculine), number (singular), and case (nominative).

Why is зал in the nominative case and not in some other case?

In Сейчас большой зал пустой, зал is the subject of the sentence:

зал = what we are talking about (the hall)
большой and пустой = properties of the hall

Subjects in Russian normally appear in the nominative case.
The adjectives that describe the subject also appear in the nominative, so we get:

большой зал (big hall – subject)
зал … пустой (the hall is empty – predicate adjective in nominative)

I’ve seen forms like зал был пустым with -ым. Why isn’t it пустым here?

There are two different patterns:

Present tense, simple description:
- Зал пустой. – The hall is empty.
  Predicate adjective in nominative.
Past (or future) with быть often using instrumental:
- Зал был пустым. – The hall was empty.
- Зал будет пустым. – The hall will be empty.

So:

Present, no быть: зал пустой (nominative).
Past/future with быть: зал был/будет пустым (instrumental is common and natural).

In your sentence, it’s present time, so пустой (nominative) is the normal form.

What is the difference between пустой and свободный?

Both can sometimes be translated as empty or free, but they focus on different things:

пустой – empty, containing nothing / no people.
- пустой зал – an empty hall (no people in it, maybe no furniture).
свободный – free, available, not occupied.
- свободное место – a free seat.
- свободный столик – a free table.

For a hall:

зал пустой – nobody is in the hall; it’s empty.
зал свободен – it’s free for use / not booked (it might or might not be physically empty, but usually it is).

In Сейчас большой зал пустой, the focus is on the fact that no one is inside; it’s empty.

Could I say Большой зал сейчас пустой or Большой зал пустой сейчас? Does word order change the meaning?

All of these are grammatical, but the default and most neutral in writing is:

Сейчас большой зал пустой.

Other orders:

Большой зал сейчас пустой.
Very natural in speech. Slightly more emphasis on big hall as the topic: “As for the big hall, right now it’s empty.”
Большой зал пустой сейчас.
Also possible, but sounds a bit more marked/emphatic. Often used in contrast:
- Большой зал пустой сейчас, а малый зал полон.
  The big hall is empty now, but the small hall is full.

The basic meaning (now the big hall is empty) stays the same; word order mostly affects emphasis and what you present as the main topic.

What exactly does сейчас mean here: “right now” or “these days”?

сейчас can mean both, depending on context:

Right now, at this moment:
- In your sentence, this is the most natural reading:
  Right now, the big hall is empty.
Nowadays / currently, in this period of time:
- Сейчас многие работают из дома. – Nowadays, many people work from home.

Without extra context, learners usually interpret Сейчас большой зал пустой as “Right now the big hall is empty.”

Does большой зал mean a big hall or the big hall?

It can mean either. Russian has no articles (a / the), so:

большой зал could be:
- a big hall (introducing it for the first time)
- the big hall (if speakers already know which hall is meant)

Context determines whether English speakers would use a or the in translation. Grammatically, Russian does not mark that difference.

Why is зал masculine, and how do I know that?

Most Russian nouns ending in a consonant are masculine. зал ends in -л, so:

зал – masculine noun.

Adjectives describing it must take masculine forms:

большой зал (not большая or большое)
пустой зал (not пустая or пустое)

So in Сейчас большой зал пустой, both adjectives are masculine singular nominative to agree with зал.

Could I drop Сейчас and just say Большой зал пустой?

Yes, absolutely:

Большой зал пустой. – The big hall is empty.

That sentence is fully correct.
The only thing you lose is the explicit reference to time now. Without сейчас, it can sound more like a general statement or a description in a particular moment that is understood from context.

Can I say Сейчас большой зал пуст instead of пустой? What is the difference?

Yes, you can:

Сейчас большой зал пуст. – perfectly correct.

пуст is the short form of the adjective пустой. Differences:

пустой (long form)
- Very common and neutral.
- Used both before and after nouns:
  - пустой зал, зал пустой.
пуст (short form)
- Used almost only in the predicate (after the noun): зал пуст.
- Often sounds a bit more formal, literary, or succinct.

In everyday speech, зал пустой is more common; зал пуст feels slightly more bookish or stylistically marked.

How would I make this sentence negative?

You negate the predicate adjective with не:

Сейчас большой зал не пустой. – Right now the big hall is not empty.

Commonly, you would add what it actually is:

Сейчас большой зал не пустой, там много людей.
Right now the big hall is not empty; there are many people there.

The не goes directly before пустой (or пуст if you use the short form: не пуст).