وضعت الطرد في الصندوق، ثم خرجت من مكتب البريد.

Because both are in the perfect/past tense in Arabic.

• وضعت = put / placed
• خرجت = went out / left

In Modern Standard Arabic, this verb form normally describes completed actions in the past. So the sentence naturally tells a sequence of things that already happened.

In Arabic, the subject is often built into the verb itself.

So وضعت can mean I put when it is read as waḍaʿtu, and خرجت can mean I went out when read as kharajtu. Arabic does not need a separate pronoun like أنا unless the speaker wants extra emphasis.

So:

• وضعت الطرد = I put the package
• خرجت من مكتب البريد = I left the post office

Without vowel marks, وضعت is written in a way that can be ambiguous.

It could be read as:

• وَضَعْتُ = I put
• وَضَعْتَ = you put to a male
• وَضَعْتِ = you put to a female
• وَضَعَتْ = she put

The same is true for خرجت.

So how do you know? Usually from:

• the context
• the translation
• or vowel marks if they are written

In normal unvowelled Arabic text, readers depend heavily on context.

Because verb-first word order is very common in Arabic.

The sentence begins with:

• وضعت الطرد...
• ثم خرجت...

This is a normal Arabic way to narrate events. English usually requires an explicit subject, so English says I put... then I left..., but Arabic can simply begin with the verb.

الطرد is the direct object of وضعت.

It is the thing that was placed. So in:

• وضعت الطرد

the action is placing, and الطرد is what got placed.

In fully vowelled formal Arabic, it would be:

• الطردَ

with the accusative ending, because it is the direct object.

في is the preposition meaning in or inside.

So:

• في الصندوق = in the box

This is a prepositional phrase telling you where the package was placed.

Also, after a preposition like في, the noun is in the genitive case in full MSA, so fully vowelled it would be:

• في الصندوقِ

ثم means then.

It shows that the second action happened after the first one, in sequence:

1. وضعت الطرد في الصندوق
2. ثم خرجت من مكتب البريد

Compared with و:

• و simply means and
• ثم means then, often with a clearer sense of order or progression

So ثم is a very natural choice here because the sentence is describing one completed action followed by another.

Because من means from.

So:

• خرجت من مكتب البريد = I went out from the post office / I left the post office

The preposition من marks the starting point or place of departure. Since the action is leaving, من is exactly the preposition you would expect.

This is an iḍāfa structure, often called a construct phrase.

It is made of:

• مكتب = office / desk
• البريد = the mail / the post

Together:

• مكتب البريد = the office of the mail
• naturally in English: the post office

A key point about iḍāfa:

• the first noun usually does not take ال
• the second noun can make the whole phrase definite

So مكتب البريد is definite even though مكتب has no ال.

Because of sun letters.

In both الطرد and الصندوق, the ل of ال is not pronounced clearly. Instead, the next consonant is doubled:

• الطرد is pronounced roughly aṭ-ṭard
• الصندوق is pronounced roughly aṣ-ṣundūq

That happens because:

• ط is a sun letter
• ص is a sun letter

But in البريد, the ل is pronounced, because ب is a moon letter:

• البريد = al-barīd

So the spelling stays the same, but the pronunciation changes depending on the first letter after ال.

A fully vowelled version would be:

وَضَعْتُ الطَّرْدَ فِي الصُّنْدُوقِ، ثُمَّ خَرَجْتُ مِنْ مَكْتَبِ الْبَرِيدِ.

Here is what those endings show:

• وَضَعْتُ = I put
• الطَّرْدَ = direct object, so accusative
• فِي الصُّنْدُوقِ = after a preposition, so genitive
• خَرَجْتُ = I went out
• مِنْ مَكْتَبِ الْبَرِيدِ = after من, so genitive

In normal everyday Arabic writing, these short vowels are usually omitted.

If the intended reading is I put... then I left..., the verbs do not show whether the speaker is male or female.

• وضعتُ = I put
• خرجتُ = I left

First-person singular past tense in Arabic does not distinguish masculine and feminine. So a man or a woman could say this exact sentence. Only context would tell you the speaker’s gender.