أرسل لي رسالة.

A careful Modern Standard Arabic pronunciation is:

أَرْسِلْ لِي رِسَالَةً
arsil lī risālatan

In normal pause at the end of the sentence, the last short ending is usually dropped, so it often sounds more like:

arsil lī risālah

A few notes:

• أرسل here is pronounced arsil, not arsala
• لي is lī
• رسالة is risālah in pause, but risālatan in full grammatical pronunciation

Word by word:

• أرسل = send!
• لي = to me / for me
• رسالة = a message / a letter

So the literal structure is:

Send to me a message.

Natural English usually says:

Send me a message.

Here, أرسل is an imperative verb, meaning a command: send!

More specifically, it is:

• 2nd person
• masculine singular
• from the verb أرسلَ = to send

So this exact form is used when speaking to one male.

Because Arabic verbs often already include the subject.

In أرسل, the hidden subject is you. So Arabic does not need to say a separate word for you here.

English:

• Send me a message.

Arabic:

• أرسل لي رسالة.

The verb itself already tells you that this is a command addressed to one person.

لي means to me or for me.

It is made of:

• لِ = to / for
• ـي = me (attached pronoun)

So:

• لي = to me

Arabic often attaches pronouns directly to prepositions, instead of using a separate word like English does.

Because رسالة means a message, not the message.

• رسالة = a message
• الرسالة = the message

So:

• أرسل لي رسالة. = Send me a message.
• أرسل لي الرسالة. = Send me the message.

The indefinite form is used when you mean any message, not a specific one already known to both people.

Starting with the verb is very normal in Arabic, especially with commands.

So أرسل لي رسالة is a completely natural Arabic structure:

• verb
  • indirect object
    • direct object

Arabic often uses verb-first order more freely than English does.

Yes, but أرسل لي رسالة is the most natural and common order here.

You may also see other orders in some contexts, but they can sound less neutral or give a different emphasis.

For example:

• أرسل لي رسالة = the most straightforward and natural
• أرسل رسالة لي = possible, but less usual in this kind of simple request

For learners, أرسل لي رسالة is the best version to use.

The form changes depending on who you are addressing.

• to one man: أرسل لي رسالة.
• to one woman: أرسلي لي رسالة.
• to more than one person: أرسلوا لي رسالة.

So the sentence you were given is specifically for one male addressee.

In fully vowelled, careful MSA, it is:

أَرْسِلْ لِي رِسَالَةً

Why?

• أَرْسِلْ = imperative verb
• لِي = preposition + attached pronoun
• رِسَالَةً = direct object, so it takes the accusative ending

That final -an sound on رسالةً is often not pronounced in normal pause at the end of the sentence, which is why you will often hear risālah instead of risālatan.

It can mean message or letter, depending on context.

In modern usage:

• رسالة can be a general message
• it can also mean a letter
• for a text message, people may say رسالة نصية

So أرسل لي رسالة usually works well for Send me a message, but context tells you what kind of message is meant.

By itself, it is a direct command: Send me a message.

That is not automatically rude, but if you want to sound more polite, you can add something like:

• من فضلك، أرسل لي رسالة. = Please send me a message.
• هل يمكنك أن ترسل لي رسالة؟ = Can you send me a message?

So أرسل لي رسالة is correct, but the polite version depends on the situation.