Sara är ansvarig för mötet på tisdag, så hon måste förbereda allt i kväll.

Both are possible, but they are not exactly the same in feel.

• är ansvarig för = is responsible for
• ansvarar för = is in charge of / takes responsibility for

In this sentence, är ansvarig för mötet sounds very natural because it describes Sara’s role or responsibility.
You could also say Sara ansvarar för mötet på tisdag, and that would also be correct.

So this is mostly a choice of expression, not a grammar rule that changes the meaning a lot.

Because ansvarig normally goes with för in Swedish:

• ansvarig för något = responsible for something
• ansvarig för mötet = responsible for the meeting

This is just a fixed combination you need to learn. Prepositions often do not match English perfectly word for word, so it is best to memorize ansvarig för as a unit.

mötet is the definite form: the meeting.
ett möte is the indefinite form: a meeting.

Here, the speaker means a specific meeting, not just any meeting, so Swedish uses the definite form:

• ett möte = a meeting
• mötet = the meeting

Swedish often marks definiteness by adding an ending to the noun, instead of using a separate word like the.

Swedish uses different prepositions for different kinds of time expressions.

• på is commonly used with days:

  • på måndag
  • på tisdag
  • på fredag

• i is commonly used with parts of the day or time periods:

  • i kväll
  • i morgon
  • i natt

So:

• på tisdag = on Tuesday
• i kväll = tonight / this evening

This is very normal Swedish usage.

Yes. Both are correct.

• i kväll
• ikväll

They mean the same thing. The two-word spelling is a little more traditional, while the one-word spelling is also very common in modern Swedish. In this sentence, i kväll is completely normal.

Because måste is a modal verb, and modal verbs in Swedish are followed by the bare infinitive, without att.

Examples:

• hon måste förbereda = she must prepare
• hon kan komma = she can come
• jag vill gå = I want to go

So:

• måste förbereda is correct
• måste att förbereda is incorrect

This is similar to English, where you say must prepare, not must to prepare.

Because after måste, Swedish uses the infinitive form of the main verb.

• måste förbereda = must prepare
• förbereder = prepares / is preparing

So in this sentence:

• måste is the finite verb
• förbereda is the infinitive that follows it

This is why förbereda is correct here.

Because så here is a coordinating conjunction meaning so, and after a coordinating conjunction, Swedish keeps normal main-clause word order:

• så hon måste ...

That means:

1. subject first: hon
2. verb second: måste

So this part works like an ordinary main clause.

Compare:

• Sara är ansvarig för mötet på tisdag, så hon måste förbereda allt i kväll.

But if a different element comes first in the clause, then inversion happens:

• I kväll måste hon förbereda allt.

There, i kväll is in first position, so the verb comes before the subject.

Because Swedish often uses the present tense for future meaning when the time is clear from the context.

In this sentence:

• på tisdag shows the meeting is in the future
• i kväll shows when the preparation has to happen

So Swedish does not need a special future form here. This is very common.

Examples:

• Jag jobbar i morgon. = I’m working tomorrow.
• Vi åker på fredag. = We’re leaving on Friday.

Also, måste describes a present obligation: right now, it is necessary for her to prepare.

Yes, allting would also work.

• allt = everything
• allting = everything

In many situations, they mean almost the same thing.
allt is very common and often sounds a little simpler and more neutral.
allting can sound a bit fuller or slightly more emphatic, depending on context.

So these are both possible:

• hon måste förbereda allt i kväll
• hon måste förbereda allting i kväll

The version with allt is very natural.

Because Swedish often puts time expressions later in the clause, especially after the object.

So this word order is very natural:

• hon måste förbereda allt i kväll

Structure:

• hon = subject
• måste = finite verb
• förbereda = infinitive
• allt = object
• i kväll = time expression

But Swedish is flexible, and you can move the time expression to the front for emphasis:

• I kväll måste hon förbereda allt.

That version emphasizes tonight more strongly.