Tiner kullandıkları sırada pencereyi açık tuttular.

Literally, it means something like at the time when they were using thinner.

• tiner = thinner / solvent
• kullandıkları = that they used / were using
• sırada = at that time / during that time

So this whole part gives the time setting for the main action.

Because Turkish often packs into one word what English expresses with several words.

kullandıkları can be broken down like this:

• kullan- = use
• -dık = a suffix that turns the verb into a clause like that ... used
• -ları = marks a plural subject here, roughly their

So kullandıkları sıra is literally something like the time that they used it / their using time, but naturally it means the time when they were using it or simply while they were using.

Turkish often does not need a separate word like English while. It frequently uses:

• a suffix construction, as in kullandıkları sırada
• or a form like kullanırken

So instead of adding a separate word, Turkish builds the idea of while/when into the grammar of the clause.

Yes. A very natural alternative is:

Tiner kullanırken pencereyi açık tuttular.

That also means They kept the window open while using thinner.

Compared with kullandıkları sırada, kullanırken is shorter and often more conversational. The original version can sound a bit more explicit or formal.

Because tiner here is an indefinite or non-specific object: using thinner in a general sense.

In Turkish, direct objects are often:

• unmarked if they are indefinite/non-specific
• marked with the accusative if they are definite/specific

So:

• tiner kullanmak = to use thinner
• tineri kullanmak = to use the thinner / a specific thinner

Because pencereyi is a definite/specific direct object: the window.

• pencere = window
• pencereyi = the window (as a direct object)

Turkish usually marks a specific direct object with the accusative ending. So pencereyi açık tuttular means they kept the window open, with a specific window in mind.

Açık tutmak is a common Turkish expression meaning to keep open.

• açık = open
• tutmak = to hold / keep
• tuttular = they kept

So pencereyi açık tuttular means they kept the window open.

This is different from açtılar, which would mean they opened it.
So the sentence is about maintaining the state of being open, not just causing it once.

It is not stated separately, because Turkish often leaves subject pronouns out when the verb already shows the subject.

Here:

• tuttular = they kept
• the -lar part shows third person plural

So English needs they, but Turkish does not have to say onlar unless the speaker wants extra emphasis or contrast.

It points to plurality, but the two cases are not exactly the same.

• In tuttular, -lar is the normal they ending on the finite verb.
• In kullandıkları, -ları is part of the subordinate-clause structure and shows the subject of that clause is plural.

So both forms help us understand that the people involved are they, but the grammar is slightly different in each word.

Because Turkish normally puts the main finite verb at the end.

This sentence follows a very typical Turkish order:

• time/background information first: Tiner kullandıkları sırada
• object and complement next: pencereyi açık
• main verb last: tuttular

So the sentence structure is very natural for Turkish. English and Turkish organize information differently here.

Tuttular is in the simple past:

• tut- = keep/hold
• -tu = past
• -lar = they

So literally it is they kept.

In English, though, the time clause is usually translated as while they were using thinner, because that sounds more natural with while. Turkish does not need to match English tense choices word-for-word here.