اول ما وصلت المكتبة، اشتريت قلم جديد.

اول ما means as soon as or the moment that. It introduces the first action in a sequence.

So:

اول ما وصلت المكتبة
= As soon as I arrived at the library

In Egyptian Arabic, this is a very common way to connect two actions.

In informal Arabic writing, people often leave out some hamzas, especially in everyday typing. So اول is very commonly written instead of the more standard spelling أول.

They mean the same thing. For a learner, it is good to recognize both.

Arabic usually does not need an explicit subject pronoun if the verb already shows who is doing the action.

Here, وصلت and اشتريت already tell you the subject is I from context, so أنا is omitted.

You could say:

أنا اول ما وصلت المكتبة، اشتريت قلم جديد

but it is usually unnecessary unless you want emphasis.

Yes. In normal Arabic writing, short vowels are not written, so forms like وصلت and اشتريت can be ambiguous.

Depending on context, وصلت could be understood as:

• I arrived
• you arrived (masculine singular)

And اشتريت could similarly mean:

• I bought
• you bought (masculine singular)

In this sentence, the meaning is understood as I arrived and I bought because of the context and the translation you were given.

This is because وصل can work like reach or arrive at and can take the destination directly.

So:

وصلت المكتبة
literally feels like I reached the library

In English, you normally say arrived at the library, but Arabic does not always need a separate preposition here.

In Egyptian, you may also hear forms with لـ in some contexts, but وصلت المكتبة is natural and understandable.

A common Egyptian-style pronunciation would be:

awwal ma wiṣilt el-maktaba, ishtareit ʔalam gedīd

A few notes:

• المكتبة is usually pronounced el-maktaba
• قلم is often pronounced ʔalam in Cairo-type Egyptian, because ق is commonly a glottal stop
• جديد is pronounced gedīd in Egyptian, not jadīd

Pronunciation can vary a bit by region and transliteration style.

In Arabic, adjectives normally come after the noun they describe.

So:

• قلم جديد = a new pen
• not new pen in the English order

This is one of the basic word-order differences between English and Arabic.

Because قلم is a masculine noun, the adjective must also be masculine.

So:

• قلم جديد = a new pen
• شنطة جديدة = a new bag

The adjective agrees with the noun in gender.

Because قلم جديد is indefinite: a new pen, not the new pen.

In Arabic, if the noun is indefinite, the adjective is also indefinite:

• قلم جديد = a new pen

If the noun is definite, the adjective must also be definite:

• القلم الجديد = the new pen

So noun and adjective match in definiteness too.

Both are in the past tense.

• وصلت = I arrived
• اشتريت = I bought

Arabic often uses the past tense this way to describe completed actions in sequence:

1. I arrived at the library
2. I bought a new pen

Yes. لما وصلت المكتبة would mean when I arrived at the library.

The difference is nuance:

• اول ما وصلت المكتبة = as soon as I arrived at the library
• لما وصلت المكتبة = when I arrived at the library

So اول ما gives a stronger sense that the second action happened immediately after the first one.

It is very easy for other Arabic speakers to understand, because the vocabulary is common and the spelling is close to standard Arabic style.

What feels especially natural for Egyptian learners is things like:

• the everyday connector اول ما
• Egyptian pronunciation such as gedīd for جديد
• omission of case endings, which is normal in spoken Arabic

So the sentence is very accessible across Arabic, but you can definitely treat it as a useful Egyptian-style sentence.