إذا كانت الفاتورة في الصندوق، أضعها في الحقيبة قبل أن نخرج.

إذا introduces a condition. Here it means if:

• إذا كانت الفاتورة في الصندوق = if the invoice/bill is in the box

A useful nuance: إذا is often used for a condition that is realistic, expected, or part of a normal sequence of events. In some contexts it can even feel close to when, but here if is the best match.

This is a very common question. In Arabic, after إذا, it is normal to use a past-form verb even when the meaning in English is present or future.

So:

• إذا كانت الفاتورة في الصندوق
  does not mean if the invoice was in the box
• it means if the invoice is in the box

This is just a normal Arabic pattern in conditional clauses. The form is past, but the meaning is not necessarily past.

Also, Arabic often uses كان and its forms to express to be in places where English just uses is/was.

Because الفاتورة is grammatically feminine.

Arabic verbs agree with their subjects, so:

• كان = masculine singular
• كانت = feminine singular

Since الفاتورة ends in ـة and is feminine, the verb must be feminine too:

• كانت الفاتورة... = the invoice was / is ...

This feminine agreement also shows up later in أضعها, where ها refers back to الفاتورة.

ها is an attached object pronoun meaning her/it (feminine singular).

So:

• أضع = I put
• أضعها = I put it

Here it refers to الفاتورة.

So the structure is:

• أضعها في الحقيبة = I put it in the bag

Because الفاتورة is feminine, the pronoun is feminine too.

The Arabic imperfect can cover several meanings depending on context, including:

• present: I put
• habitual: I usually put
• near future: I will put

In a sentence like this, after a condition introduced by إذا, English often uses a future translation:

• If the invoice is in the box, I’ll put it in the bag...

But Arabic still naturally uses أضع.

So أضعها could be understood as:

• I put it
• I will put it

Context decides which English wording sounds best.

It means before we leave or literally before we go out.

Breakdown:

• قبل = before
• أن = a particle used before a verb
• نخرج = we leave / we go out

So:

• قبل أن نخرج = before we leave

This is a very common Arabic pattern:

• قبل أن + verb
• بعد أن + verb

For example:

• قبل أن نأكل = before we eat
• بعد أن نصل = after we arrive

After words like قبل when a full verbal clause follows, Arabic normally uses أن before the verb.

So Arabic says:

• قبل أن نخرج

not just

• قبل نخرج

Grammatically, أن causes the following imperfect verb to be in the subjunctive. In fully vowelled Arabic, that would be:

• نخرجَ

But in normal unvowelled writing, it is written the same way:

• نخرج

So learners often do not see the change unless vowel marks are added.

Both are possible.

The root خرج literally has the idea of going out / coming out. But in natural English, depending on context, it is often translated as:

• leave
• go out

So here:

• قبل أن نخرج can be before we leave
• or more literally before we go out

Both are correct; before we leave is probably the more natural translation in this sentence.

If you want the grammar breakdown:

• الفاتورة is the subject of كانت
• في الصندوق is the predicate/complement telling where it is
• الحقيبة comes after في, so it is inside a prepositional phrase
• الصندوق also comes after في, so it is inside a prepositional phrase

In fully grammatical terms:

• الفاتورةُ = nominative
• الصندوقِ = genitive after في
• الحقيبةِ = genitive after في

So the sentence is built very regularly.

A fully vowelled version could be written as:

إذا كانتِ الفاتورةُ في الصندوقِ، أضعُها في الحقيبةِ قبلَ أن نخرجَ.

A few things to notice:

• الفاتورةُ has ـُ because it is nominative
• الصندوقِ and الحقيبةِ have ـِ because they come after في
• أضعُ has ـُ because it is indicative
• قبلَ often appears with ـَ
• نخرجَ has ـَ because of أن

In everyday printed Arabic, most of these short vowels are omitted.

Because ص is a sun letter.

With the definite article ال, if the next letter is a sun letter, the l sound assimilates to that letter. So:

• الصندوق is written with ال
• but pronounced aṣ-ṣundūq

By contrast, الحقيبة begins with ح, which is a moon letter, so the l is pronounced:

• al-ḥaqībah

So this sentence gives you one example of each pattern:

• الصندوق → sun-letter pronunciation
• الحقيبة → moon-letter pronunciation