لما فتحت الرسالة، رديت عليها على طول.

Here, لما means when. It introduces a time clause: when I opened the message...

In Egyptian Arabic, لما + past verb is a very common way to talk about something that happened when another action happened. In this sentence, it sets up the first event, and the second clause tells you what happened next.

Yes, by itself فتحت can mean:

• I opened
• you (masc.) opened
• she opened

Arabic often depends on context to decide which one is meant. Here, the next verb is رديت, which clearly means I replied, so the sentence is understood as I opened ... I replied ...

Because the sentence is describing two completed actions in sequence:

1. opening the message
2. replying right away

In Egyptian Arabic, this is very normal: لما + past verb, then another past verb for the next completed action. It works a lot like When I opened the message, I replied immediately in English.

Because Arabic verbs already show the subject. The form فتحت already includes I, and رديت also includes I.

So أنا is usually omitted unless you want emphasis or contrast. For example, you might say أنا رديت عليها if you specifically want to stress I replied to it.

Because ر is a sun letter. In Arabic, when الـ comes before a sun letter, the l sound is absorbed into the next consonant.

So:

• written: الرسالة
• pronounced: er-risāla / ir-risāla depending on how you transliterate it

This is a pronunciation rule only; the spelling still stays الرسالة.

رديت is the Egyptian Arabic past-tense form meaning I replied.

It comes from the verb رد. In Egyptian, this verb is very commonly used for to reply / answer back. So:

• ردّ = he replied
• رديت = I replied

This is a normal everyday Egyptian form. A learner who knows Modern Standard Arabic may expect something a little different, but رديت is the natural colloquial Egyptian choice here.

Because the verb ردّ in Egyptian is commonly used with the preposition على when it means reply to someone or something.

So:

• رديت عليها = I replied to it / her
• literally: I replied on it/her

This is just how the verb works in Arabic. English says reply to, and Egyptian Arabic uses ردّ على.

Because رسالة is a feminine noun in Arabic. So when you refer back to it with a pronoun, you use the feminine singular form:

• عليها = على + ها
• ها = it / her (feminine singular)

Even though message is not feminine in English, Arabic grammar treats رسالة as feminine, so the pronoun has to match that.

They are doing two different jobs:

• عليها: part of the verb pattern ردّ على = reply to
• على طول: a fixed expression meaning immediately / right away

So even though both contain على, they are not repetitions of the same thing in meaning.

Here, على طول means right away, immediately, or straight away.

It is a very common Egyptian expression. In other contexts, it can also mean something like all the time or continuously, but in this sentence the meaning is clearly immediately.

So the sense is: as soon as the message was opened, the reply happened without delay.

A common Egyptian-style pronunciation would be:

lammā fataḥt er-risāla, raddēt ʿalēha ʿala ṭūl

A few things to notice:

• لما = lammā
• الرسالة sounds like er-risāla because of the sun-letter rule
• رديت is pronounced like raddēt
• عليها is often heard as ʿalēha

Yes. In many situations, أول ما can also work and would mean something like as soon as.

So a natural alternative would be:

أول ما فتحت الرسالة، رديت عليها على طول.

That version can feel a little more explicitly like the moment I opened the message. But لما in your original sentence is completely natural and common.