أضع الطابع على الطرد، ثم أضع الطرد في الصندوق.

It has two short verbal clauses joined by ثم.

• أضع الطابع على الطرد
  = verb + direct object + prepositional phrase
• ثم أضع الطرد في الصندوق
  = then + verb + direct object + prepositional phrase

A quick breakdown:

• أضع = I put / place
• الطابع = the stamp
• على = on
• الطرد = the parcel/package
• ثم = then
• في = in
• الصندوق = the box

So the sentence is built in a very normal Arabic way.

أضع is the first-person singular imperfect form of the verb وضع, which means to put or to place.

So أضع means:

• I put
• I place
• depending on context, also I am putting

A useful thing to notice: the past tense is وضعتُ = I put/placed, but in the imperfect the initial و of the root does not appear here, so you get أضع. This is a normal pattern with some verbs.

Because Arabic usually does not need a separate subject pronoun when the verb already shows the subject.

In أضع, the prefix أ- tells you the subject is I.

So:

• أضع = I put
• أنا أضع = I put

Both are possible, but أنا is optional. You would add أنا mainly for emphasis, contrast, or clarity.

Because the sentence is talking about specific things:

• الطابع = the stamp
• الطرد = the parcel
• الصندوق = the box

Arabic uses الـ to mark definiteness, like English the.

Without الـ, the nouns would be indefinite:

• طابع = a stamp
• طرد = a parcel
• صندوق = a box

In this sentence, the speaker seems to mean particular, known items, so the definite article is natural.

Arabic often repeats the noun for clarity, especially when more than one thing has just been mentioned.

Here we have both:

• الطابع = the stamp
• الطرد = the parcel

If you said ثم أضعه في الصندوق, the -ه means it/him, but it could momentarily make the listener ask: the stamp or the parcel?

So repeating الطرد makes the meaning very clear:

• ثم أضع الطرد في الصندوق = then I put the parcel in the box

A pronoun version is possible, but repetition is very natural and often clearer.

ثم means then and shows a sequence of actions, usually with a clear step after the first one.

So:

• ثم = then / after that

If you used و, it would just mean and, without emphasizing the sequence as clearly.

Compare:

• أضع الطابع على الطرد، ثم أضع الطرد في الصندوق
  = I put the stamp on the parcel, then I put the parcel in the box.
• أضع الطابع على الطرد، وأضع الطرد في الصندوق
  = I put the stamp on the parcel, and I put the parcel in the box.

Both are possible, but ثم is better if you want to highlight the order of steps.

Because the relationships are different:

• على الطرد = on the parcel
• في الصندوق = in the box

A stamp is attached on the surface of the parcel, so على is used.

A parcel goes inside a box, so في is used.

This is similar to English, but it is still worth memorizing as part of the phrase:

• وضع الطابع على الطرد = put the stamp on the parcel
• وضع الطرد في الصندوق = put the parcel in the box

In these words, the ل of الـ is not pronounced normally, because ط and ص are sun letters.

So the pronunciation is roughly:

• الطابع → aṭ-ṭābiʿ
• الطرد → aṭ-ṭard
• الصندوق → aṣ-ṣundūq

The spelling still keeps الـ, but the pronunciation assimilates to the next consonant.

So the whole sentence in careful pronunciation is roughly:

aḍaʿu aṭ-ṭābiʿa ʿalā aṭ-ṭardi, thumma aḍaʿu aṭ-ṭarda fī ṣ-ṣundūqi

With full case endings, the sentence is:

أضعُ الطابعَ على الطردِ، ثم أضعُ الطردَ في الصندوقِ.

Why those endings?

• أضعُ: ending -u because the imperfect verb is in the normal indicative form
• الطابعَ: accusative because it is the direct object
• الطردِ after على: genitive because it comes after a preposition
• الطردَ in the second clause: accusative because it is again the direct object
• الصندوقِ after في: genitive because it comes after a preposition

In ordinary Arabic writing, these short vowels are usually not written.

Yes, this is very normal.

Arabic often begins a sentence with the verb, especially in straightforward action statements:

• أضع الطابع... = I put the stamp...

This is called a verbal sentence.

You could also say:

• أنا أضع الطابع على الطرد...

That is also correct, but it gives more emphasis to I. The version without أنا is the more neutral, natural one in many contexts.

It can cover more than one English idea, because the Arabic imperfect is broader than the English present tense.

Depending on context, أضع can mean:

• I put
• I am putting
• sometimes even I will put

In this sentence, because of ثم, it sounds like a sequence of actions or steps:

• I put the stamp on the parcel, then I put the parcel in the box

If the speaker is describing a routine, it could also mean something like:

• I put the stamp on the parcel, then I put the parcel in the box

So English translation depends on context, but the Arabic form itself is completely normal here.

No. طابع can have several meanings depending on context, including:

• stamp
• character
• nature
• impression

But in this sentence, because it is used with على الطرد, it clearly means a postal stamp.

So context tells you which meaning is intended.