روی دیوار اتاق یک آینه بزرگ بود.

Persian often puts a location phrase first when setting the scene.
So روی دیوار اتاق means on the wall of the room, and then the sentence tells you what was there:

روی دیوار اتاق یک آینه بزرگ بود.
= On the wall of the room, there was a big mirror.

This word order is very natural in Persian. English can do this too, but English more often says There was a big mirror on the wall of the room.

روی means on, on top of, or upon.

In this sentence:

• روی دیوار = on the wall

It is a very common preposition.
You will often hear it in phrases like:

• روی میز = on the table
• روی زمین = on the ground
• روی صندلی = on the chair

In pronunciation, روی is usually ru-ye.

Persian usually connects nouns with a feature called ezafe.
So دیوار اتاق is really pronounced:

divâr-e otâq
= the wall of the room / the room’s wall

The little -e sound between the nouns shows the relationship, like English of.

Important point:
The ezafe is often not written in normal spelling, especially after consonants, but it is still pronounced.

So:

• written: دیوار اتاق
• pronounced: divâr-e otâq

Persian does not have a definite article like English the.

So a noun can be understood as the, a, or just a general noun depending on context.

Here, اتاق is naturally understood as the room, and دیوار اتاق as the wall of the room or the room wall.

That is completely normal in Persian.

یک literally means one, but it is also commonly used like the English a/an.

So:

• یک آینه بزرگ = a big mirror

Depending on context, یک may emphasize one, but here it mainly works as an indefinite marker.

In everyday speech, یک is often pronounced یه.

So this sentence is often said as:

روی دیوار اتاق یه آینه بزرگ بود.

That sounds more conversational.

In Persian, adjectives usually come after the noun, not before it.

So:

• آینه بزرگ = big mirror

More precisely, there is usually an ezafe connection here too:

• pronounced: âyene-ye bozorg

So the full noun phrase is really:

• یک آینه بزرگ
• pronounced: yek âyene-ye bozorg
• meaning: a big mirror

This is the normal Persian pattern:

• کتاب خوب = good book
• خانه بزرگ = big house
• ماشین جدید = new car

Yes. There is normally an ezafe there as well.

So:

• آینه بزرگ is pronounced âyene-ye bozorg

Because آینه ends in ه, the ezafe may sometimes be written more clearly as:

• آینهٔ بزرگ
• or آینه‌ی بزرگ

But in many ordinary texts, writers leave that marking out, and learners are expected to know it from grammar and context.

So even if you see:

• آینه بزرگ

you should usually understand it as:

• آینهِ بزرگ / آینه‌ی بزرگ
• big mirror

Persian is generally a verb-final language, so the main verb often comes at the end of the sentence.

Here:

• بود = was

So the structure is roughly:

• روی دیوار اتاق = on the wall of the room
• یک آینه بزرگ = a big mirror
• بود = was

Literally, Persian is closer to:

On the wall of the room a big mirror was.

That sounds odd in English, but it is normal in Persian.

Persian often expresses there was / there is simply by using the thing + location + form of to be, without a separate dummy word like English there.

So:

• روی دیوار اتاق یک آینه بزرگ بود
• literally: On the wall of the room, a big mirror was
• natural English: There was a big mirror on the wall of the room

English needs there was in this kind of sentence, but Persian does not.

Because آینه is singular.

• آینه = mirror
• بود = was

If the noun were plural, you would usually use بودند:

• روی دیوار اتاق چند آینه بزرگ بودند
  = There were several big mirrors on the wall of the room

So the verb agrees with the subject in number.

A careful pronunciation would be:

ru-ye divâr-e otâq yek âyene-ye bozorg bud

In more natural everyday speech, many speakers might say:

ru-ye divâr-e otâq ye âyene-ye bozorg bud

A few useful pronunciation notes:

• روی = ru-ye
• دیوار اتاق = divâr-e otâq
• یک often sounds like ye
• آینه بزرگ = âyene-ye bozorg
• بود = bud

So even though the written sentence does not show every ezafe, the spoken sentence does.

Yes. That is a perfectly reasonable translation.

Possible natural English translations include:

• There was a big mirror on the wall of the room.
• A big mirror was on the wall of the room.
• On the wall of the room there was a big mirror.

They all match the Persian sentence well. The exact English wording depends on style, but the Persian structure is the same.

It is natural and standard Persian.

The only slightly more formal-looking part is یک instead of conversational یه. In speech, many people would say:

روی دیوار اتاق یه آینه بزرگ بود.

But the written sentence you were given is completely normal and idiomatic.