في مكتب البريد سألت الموظفة عن الطابع الذي أحتاج إليه للطرد.

Because مكتب البريد is an iḍāfa construction, literally office of the mail.

In an iḍāfa:

• the first noun usually does not take الـ
• the second noun can be definite, and that makes the whole phrase definite

So:

• مكتب بريد = a post office / a mail office
• مكتب البريد = the post office

After في, the phrase is in the genitive, so in full vocalization it would be فِي مَكْتَبِ الْبَرِيدِ.

It is the past tense of سأل = to ask.

In normal unvocalized Arabic writing, سألت can represent several forms, such as:

• سَأَلْتُ = I asked
• سَأَلْتَ = you asked (masculine singular)
• سَأَلْتِ = you asked (feminine singular)
• سَأَلَتْ = she asked

So by itself, سألت is ambiguous in writing. You know which one is meant from:

• context
• the translation/meaning already provided
• or full vowel marks, if they are written

الموظفة means the female employee / the female clerk.

The ending ـة usually marks a feminine noun. Compare:

• الموظف = the male employee
• الموظفة = the female employee

In this sentence, it is the person being asked:

• سألت الموظفة = I asked the female employee

So it is the direct object of سأل.

Because Arabic commonly says:

• سأل شخصًا عن شيء = to ask someone about something

So the pattern here is:

• سألت الموظفة = I asked the employee
• عن الطابع = about the stamp

This is a very common Arabic structure. English and Arabic do not always match word-for-word, so it is useful to learn سأل ... عن ... as a set pattern.

Here الطابع means the stamp, specifically a postage stamp.

The word can have other meanings in other contexts, but in a sentence about a post office and a package, stamp is the natural meaning.

It is definite because of الـ, and then it is further described by the relative clause:

• الذي أحتاج إليه = that/which I need

So the phrase means:

• the stamp that I need

الذي is the relative pronoun that / which / who for a masculine singular noun.

Its antecedent here is الطابع, which is masculine singular, so الذي is the correct form.

Compare:

• الذي = masculine singular
• التي = feminine singular
• الذين = masculine plural (human)
• other forms exist as well

So:

• الطابع الذي أحتاج إليه = the stamp that I need

Because the verb احتاج normally takes the preposition إلى.

You say:

• أحتاج إلى الطابع = I need the stamp

When the noun is replaced by a pronoun, the pronoun attaches to the preposition:

• إلى + ه = إليه = to it / to him

So:

• الطابع الذي أحتاج إليه literally means the stamp that I need to it
• but in natural English: the stamp that I need

This is a very important pattern:

• احتاج إلى = to need

إليه is made of:

• إلى = to
• ـه = him / it

So literally it means to him or to it.

Because الطابع is a masculine singular noun, Arabic uses the masculine singular pronoun ـه to refer back to it.

For non-human things, Arabic still uses grammatical gender:

• masculine noun → ـه
• feminine noun → ـها

So here إليه refers back to الطابع.

للطرد means for the package / for the parcel.

It is:

• لِـ = for
• الطرد = the package / parcel

So together:

• للطرد = for the package

This tells you what the stamp is for:

• not just any stamp
• but the stamp needed for the package

Because it combines:

• the preposition لِـ
• with the definite article الـ

So:

• لِ + الطرد → للطرد

This is normal Arabic spelling.

Also, because ط is a sun letter, the ل of الـ is not pronounced separately. So للطرد is pronounced roughly liṭ-ṭard.

Because ط is a sun letter.

With sun letters, the ل in الـ is written but assimilates in pronunciation. So:

• الطابع is written with ال
• but pronounced aṭ-ṭābiʿ, not al-ṭābiʿ

The same thing happens in:

• الشمس → ash-shams
• الطالب → aṭ-ṭālib

So the spelling stays the same, but the pronunciation changes.

That fronted phrase sets the scene: At the post office, ...

Arabic often moves time/place expressions to the beginning for context or emphasis. So this word order is very natural:

• في مكتب البريد سألت الموظفة...

You could also say something like:

• سألت الموظفة في مكتب البريد عن الطابع...

The basic meaning stays the same, but the original version gives the location first, which sounds natural and organized.

A fully vocalized version would be:

فِي مَكْتَبِ الْبَرِيدِ سَأَلْتُ الْمُوَظَّفَةَ عَنِ الطَّابِعِ الَّذِي أَحْتَاجُ إِلَيْهِ لِلطَّرْدِ.

A few useful things to notice:

• مكتبِ is genitive after في
• البريدِ is genitive as the second term of the iḍāfa
• الموظفةَ is accusative as the direct object
• الطابعِ is genitive after عن
• أحتاجُ is present tense
• إليه contains the pronoun referring back to الطابع
• للطردِ is genitive after لِـ

In ordinary Arabic writing, most of these short vowels are not written, so learners often have to infer them from grammar and context.