Learn Hardcore Swedish: Jag måste skicka meddelandet snabbt.

Because måste is a modal auxiliary verb in Swedish. Modal verbs like måste, kan, vill, ska, bör and får are followed by the bare infinitive without att. For example: Jag kan simma, not Jag kan att simma.

Could I say jag behöver skicka meddelandet snabbt instead of jag måste skicka meddelandet snabbt?

Yes, you can. behöva means “need to” and måste means “must” or “have to.” Both express necessity, but måste is stronger (more like an obligation) while behöver feels milder (more like a personal need). In many contexts they’re interchangeable:

Jag behöver skicka meddelandet. (I need to send the message.)
Jag måste skicka meddelandet. (I have to/must send the message.)

Why is it meddelandet instead of meddelande?

Because meddelandet is the definite form (“the message”). Swedish marks definiteness with a suffix on the noun:

ett meddelande = a message
meddelandet = the message

Why is it snabbt and not snabb?

snabb is an adjective (describing a noun), while snabbt is its adverb form (describing a verb). Here you’re modifying skicka (how you send), so you need the adverb snabbt. Many Swedish adjectives become adverbs with -t: lång → långt, hög → högt, snabb → snabbt.

Can I use fort instead of snabbt, like Jag måste skicka meddelandet fort?

Yes. fort also means “quickly” and is very common in spoken Swedish. That sentence is perfectly natural: Jag måste skicka meddelandet fort.

Can I move snabbt to another position, for example Jag måste snabbt skicka meddelandet?

Absolutely. Both word orders are correct:

Jag måste skicka meddelandet snabbt.
Jag måste snabbt skicka meddelandet.
The first emphasizes sending the message quickly; the second is more like “I have to quickly send the message.”

Why is the word order Jag – måste – skicka – meddelandet – snabbt? Can I change it?

Swedish main clauses follow the V2 rule: the finite verb (måste) must be in second position. Here Jag (subject) is first, then måste, then the rest. You can front other elements for emphasis (time, manner, etc.), but måste stays second. For example:
Snabbt måste jag skicka meddelandet.

Can I drop jag and just say Måste skicka meddelandet snabbt?

In very informal speech or in headlines/notes you might omit jag and say Måste skicka meddelandet snabbt, but in standard Swedish you normally include the subject. Without jag it sounds like a headline or a quick reminder rather than a full sentence in conversation.