الرسالة وصلت بعد ثانية.

A common Egyptian pronunciation is:

er-risāla wiṣlit baʿd sānya

A few notes:

• الرسالة is pronounced er-risāla, not al-risāla
• وصلت is often pronounced wiṣlit or woslet/weslet depending on the speaker
• ثانية in Egyptian is usually sānya, not thāniya

So the sentence sounds much more Egyptian in speech than the spelling alone may suggest.

Because ر is a sun letter.

In Arabic, the الـ definite article changes its pronunciation before certain consonants. With sun letters, the l sound of al- disappears and the next consonant gets emphasized.

So:

• الرسالة becomes
• er-risāla / ar-risāla

The writing does not change, but the pronunciation does.

Because the subject الرسالة is grammatically feminine singular.

The verb وصلت means arrived or has arrived here, and the -ت marks 3rd person feminine singular in the past tense.

So:

• وصل = he arrived
• وصلت = she arrived / it arrived (for a feminine noun)

Since رسالة is feminine, Arabic uses the feminine verb form:

• الرسالة وصلت = the message arrived

It is the past tense.

In this sentence, وصلت means:

• arrived
• or in natural English, sometimes has arrived, depending on context

Arabic past tense often covers both simple past and present perfect meanings, so context tells you which English translation sounds best.

Both are possible.

In Egyptian Arabic, subject + verb order is very common in everyday speech:

• الرسالة وصلت

You can also say:

• وصلت الرسالة

That version is also correct and natural.
Very roughly:

• الرسالة وصلت can feel a bit more like the message arrived
• وصلت الرسالة can feel a bit more like arrived the message / the message has arrived

But in normal conversation, both are fine.

Literally, it means:

• بعد = after
• ثانية = a second

So بعد ثانية means:

• after a second
• one second later

In natural English, depending on context, this can also come across as:

• after one second
• a second later

Because in Egyptian Arabic, the letter ث is very often pronounced s instead of the Standard Arabic th sound.

So:

• Standard Arabic: thāniya
• Egyptian Arabic: sānya

This is a very common feature of Egyptian pronunciation.
So if you are learning Egyptian, sānya is the form you would usually say.

Because Arabic does not use a separate word like English a/an.

An indefinite noun can appear by itself and mean a or an.

So:

• ثانية = a second

In very formal Standard Arabic, this indefiniteness can be shown with case endings in pronunciation, but in Egyptian Arabic those endings are not used.

That is why ثانية on its own is enough to mean a second.

Yes, it can.

ثانية can mean:

• a second (the time unit)
• or second (feminine ordinal), as in the second one

But in this sentence, after بعد, the meaning is naturally understood as the time unit:

• after a second

So here there is no real confusion.

The written sentence works in both, but the spoken pronunciation makes the difference clearer.

In Egyptian Arabic, you would typically say:

• er-risāla wiṣlit baʿd sānya

In Standard Arabic, a full formal pronunciation would be more like:

• ar-risālatu waṣalat baʿda thāniyatin

So the spelling is compatible with both, but the Egyptian learner should focus on the colloquial pronunciation and the lack of formal case endings.