Verniği sürdüğümüz sırada cila hazırlandı.

Because vernik is the dictionary form, but here it is the direct object of sürmek (to apply / spread / smear), so it takes the accusative ending.

• vernik = varnish
• verniği = the varnish

Two things happen:

1. -i is added for the accusative.
2. The final k in vernik softens to ğ before a vowel.

So:

• vernik → verniği

This is very common in Turkish:

• renk → rengi
• çocuk → çocuğu
• kitap → kitabı (here p softens to b)

In this sentence, verniği sürmek means to apply the varnish.

Sürdüğümüz comes from the verb sürmek.

Here it means something like:

• that we applied
• that we were applying

It is built like this:

• sür- = verb stem, apply / spread
• -dük / -düğ- = a participle-type ending that turns the verb into a clause meaning that ...
• -ümüz = our / we

So sürdüğümüz literally means:

• that we did
• or more naturally here, that we were applying

In this sentence, it modifies the time expression:

• Verniği sürdüğümüz sırada = during the time when we were applying the varnish

This structure is very common in Turkish:

• geldiğim zaman = when I came
• yaptığımız sırada = while we were doing
• konuştuğun anda = at the moment you speak / when you speak

Because Turkish does not normally use a finite verb like sürdük directly before a noun such as sıra.

To say when we did X or while we were doing X, Turkish usually turns the verb into a verbal adjective / clause first.

So:

• sürdük = we applied
• sürdüğümüz = that we applied / that we were applying

Then that clause can combine with sırada:

• sürdüğümüz sırada = during the time that we were applying

This is much more natural Turkish grammar than trying to place a normal conjugated verb directly before sırada.

Here sırada means during the time / at the time / while.

It comes from:

• sıra = row, turn, order, sequence
• sırada = in the sequence / at that time

In this pattern, it works as a time expression:

• geldiğim sırada = when I came
• uyuduğu sırada = while he/she was sleeping
• çalıştığımız sırada = while we were working

So:

• Verniği sürdüğümüz sırada = while we were applying the varnish

It is one of several Turkish ways to express when / while.

Yes, very possibly.

A similar version would be:

• Verniği sürerken cila hazırlandı.

This also means:

• While we were applying the varnish, the polish was prepared.

The difference is mostly one of style and structure:

• sürerken is shorter and often feels more direct.
• sürdüğümüz sırada is a bit more expanded and can sound slightly more formal or narrative.

Both are natural. Turkish often has more than one way to say while:

• sürerken
• sürdüğümüz sırada
• sometimes sürdüğümüz esnada

Because hazırlanmak means to be prepared.

Here:

• hazırla- = to prepare
• hazırlan- = to be prepared / to get prepared
• hazırlandı = was prepared / got prepared

So:

• cila hazırlandı = the polish was prepared

This is different from:

• cilayı hazırladık = we prepared the polish

The sentence does not say who prepared it. Turkish often uses the passive when the doer is unknown, unimportant, or simply not mentioned.

Hazırlandı is in the simple past (also called the definite past in Turkish).

Structure:

• hazırlan- = be prepared
• -dı = past tense

So it means:

• was prepared
• got prepared

In context, English may translate it as:

• the polish was prepared
• the polish got prepared
• sometimes even the polish was being prepared, depending on context, though the Turkish form itself is simple past

Turkish tense usage does not always match English tense usage one-to-one.

Because Turkish normally places the verb at the end of the clause.

So the basic order is often:

• subject / object / other information + verb

Here:

• cila = the polish
• hazırlandı = was prepared

So:

• cila hazırlandı

That is the normal Turkish order.

If you changed the order to hazırlandı cila, it would sound unusual in ordinary prose unless there were some special stylistic reason.

Not exactly.

In many everyday contexts, the two can both refer to surface-finishing materials, so translations may overlap a bit, but they are not always the same thing.

Very roughly:

• vernik = varnish / lacquer
• cila = polish / finishing polish / shine-producing treatment

So in this sentence, they are treated as two different materials or stages.

A learner does not need to worry too much about the technical distinction every time, but it is useful to know they are not simply two forms of the same word.

-ümüz shows that the action belongs to we / our.

In this kind of Turkish clause, the person is often marked with a possessive-type ending rather than with a normal finite verb ending.

Compare:

• geldiğim zaman = when I came
• geldiğin zaman = when you came
• geldiği zaman = when he/she came
• geldiğimiz zaman = when we came

So:

• sürdüğümüz = that we applied / that we were applying

This is a very important Turkish pattern. English speakers often find it unusual at first because Turkish packs the subject information into a form that behaves more like a noun clause than a regular English-style finite clause.

No. It is a time clause / adverbial phrase.

The main part of the sentence is:

• cila hazırlandı = the polish was prepared

That is the core clause.

The first part:

• Verniği sürdüğümüz sırada

tells us when the main action happened.

So the structure is:

• Time expression: while we were applying the varnish
• Main clause: the polish was prepared

Yes. Sürmek is a very common Turkish verb with several meanings, and one of them is:

• to spread / apply onto a surface

For example:

• krem sürmek = to apply cream
• boya sürmek = to apply paint
• vernik sürmek = to apply varnish

So in this sentence, verniği sürmek is exactly the natural thing to say for putting varnish onto something.

Because Turkish usually expresses that idea through verb forms and clause structure, not with a separate verb like English were.

English often needs:

• we were applying

Turkish can express the same idea more compactly through a form like:

• sürdüğümüz sırada
• or sürerken

So the sense of ongoing action in the background comes from the whole construction, especially the time clause, rather than from a separate word meaning were.