العد التنازلي يبدأ والحجرة المخفية هنا.

Yes. العد التنازلي is the standard Arabic expression for countdown.

Literally:

• العدّ = counting
• التنازلي = descending / downward

So the phrase literally means the downward counting, which is how Arabic says countdown.

Because التنازلي is an adjective describing العد.

In Arabic, adjectives usually match the noun they describe in:

• definiteness
• gender
• number
• case

So if the noun is definite, the adjective must also be definite.

Here:

• العد = the counting
• التنازلي = the descending

Together: العد التنازلي = the countdown

If it were indefinite, you would expect something like عدّ تنازلي = a countdown / countdown-like counting.

The subject is العد التنازلي.

So:

• العد التنازلي = the countdown
• يبدأ = begins / starts

The verb is 3rd person masculine singular, because العد is grammatically masculine singular.

So Arabic is literally saying:

The countdown begins.

Both are possible, but they feel slightly different.

• يبدأ العد التنازلي = more straightforward verbal style: The countdown begins
• العد التنازلي يبدأ = puts focus on the countdown first

The version in your sentence can sound a bit more dramatic or emphatic, which fits suspenseful or narrative language well.

So this is not wrong at all; it is just a stylistic choice.

Because in Arabic, the present-tense verb to be is usually omitted.

So:

• الحجرة المخفية هنا literally = the hidden chamber here
• natural English = The hidden chamber is here

This is completely normal in Arabic.

If you wanted past tense, you would use a verb:

• كانت الحجرة المخفية هنا = The hidden chamber was here

Because الحجرة is feminine, and the adjective must agree with it.

So:

• حجرة = a feminine noun
• مخفية = feminine singular form of hidden
• المخفية = the hidden

Arabic adjectives agree with the nouns they describe, so you get:

• الحجرة المخفية = the hidden chamber

If the noun were masculine, you would use المخفي instead.

It comes from the verb أخفى = to hide.

The adjective مخفي / مخفية means hidden.

So:

• مخفي = hidden (masculine)
• مخفية = hidden (feminine)

In this sentence, it works as a descriptive adjective attached to الحجرة.

Both can refer to a room, but they are not always identical in tone.

• غرفة is the more common everyday word for room
• حجرة can sound a bit more formal, literary, or like chamber

So for something like the hidden chamber, الحجرة is a good choice because it can sound a bit more dramatic or story-like.

هنا means here.

In الحجرة المخفية هنا, it acts as the predicate of place, so the whole clause means:

The hidden chamber is here.

Arabic often puts this kind of location word after the noun phrase, so this order is very normal.

A careful full pronunciation with case endings would be approximately:

al-ʿaddu t-tanāzuliyyu yabdaʾu wa-l-ḥujratu l-makhfiyyatu hunā

A simpler reading without focusing on full case endings would be:

al-ʿadd at-tanāzulī yabdaʾ wal-ḥujra al-makhfiyya hunā

A few pronunciation notes:

• التنازلي is pronounced at-tanāzulī, because ت is a sun letter
• العدّ has a doubled d sound
• يبدأ contains a hamza at the end: yabdaʾ

Yes. It is understandable and natural, especially in a dramatic or narrative context.

It sounds a little elevated because of:

• حجرة instead of the more everyday غرفة
• the subject-first order العد التنازلي يبدأ

So it feels suitable for suspense, storytelling, or dramatic narration.