Jag vill undvika mer stress, så i kväll ska jag lägga mig tidigt och låta mobilen vara avstängd.

This is because Swedish main clauses follow the V2 rule: the finite verb must come in the second position.

In the clause i kväll ska jag lägga mig tidigt, the first position is taken by i kväll. That means the finite verb ska must come next, before the subject jag.

So the pattern is:

• I kväll = first position
• ska = second position
• jag = subject after the verb

If you started with the subject instead, you would say:

• Jag ska lägga mig tidigt i kväll.

Both are correct, but the word order changes because of the V2 rule.

Yes. Both i kväll and ikväll are used.

• i kväll = two words
• ikväll = one word

They mean the same thing: tonight.

The two-word spelling can feel a little more formal or traditional in some contexts, but both are normal Swedish.

Here ska shows a planned future action or intention.

So i kväll ska jag lägga mig tidigt means something like:

• tonight I’m going to go to bed early
• tonight I will go to bed early

In Swedish, ska often expresses:

• intention
• plan
• something expected to happen

It is very common for talking about the future, but it is not a separate future tense in the same way English sometimes talks about future forms.

Yes. The ska covers both actions.

So this part:

• ska jag lägga mig tidigt och låta mobilen vara avstängd

means:

• I’m going to go to bed early and leave my phone switched off

You do not need to repeat ska before the second infinitive. Swedish often works like English here:

• I’m going to go to bed early and leave my phone off

Because after certain verbs, Swedish uses the bare infinitive.

Two important cases in this sentence are:

• ska lägga
• låta vara

After modal verbs like ska, kan, vill, måste, you normally do not use att.

After låta in this structure, you also do not use att.

So:

• jag ska lägga mig
• låta mobilen vara avstängd

not:

• jag ska att lägga mig
• låta mobilen att vara avstängd

Lägga sig is a reflexive verb expression meaning to go to bed or literally to lay oneself down.

The reflexive pronoun changes with the subject:

• jag lägger mig
• du lägger dig
• han/hon lägger sig
• vi lägger oss

In this sentence, the subject is jag, so the reflexive pronoun is mig.

This is a very common expression:

• lägga sig tidigt = go to bed early

This is a very common learner question.

• lägga sig = to lie down / go to bed, focusing on the action of changing position
• ligga = to be lying, focusing on the state

So:

• Jag ska lägga mig tidigt. = I’m going to go to bed early.
• Jag ligger i sängen. = I am lying in bed.

Swedish often distinguishes between the action and the resulting state.

Because tidigt is being used adverbially here.

• tidig = early, as an adjective
• tidigt = early, as an adverb

In lägga mig tidigt, it describes how/when the person will go to bed, so English would also use the adverb early.

Compare:

• en tidig kväll = an early evening
  Here tidig describes a noun.

• jag lägger mig tidigt = I go to bed early
  Here tidigt describes the action.

It means to leave the phone switched off or to let the phone remain switched off.

This structure is:

• låta + object + infinitive

So here:

• låta mobilen vara avstängd

literally means something like:

• let the phone be switched off

In natural English, that often becomes:

• leave the phone switched off
• keep the phone off

This is slightly different from stänga av mobilen, which means turn off the phone.
Låta mobilen vara avstängd emphasizes the state continuing, not just the action of switching it off.

Because avstängd agrees with mobilen, which is a common gender noun.

• en mobil
• mobilen

When used predicatively after vara, the adjective/participle agrees with the noun:

• common gender singular: avstängd
• neuter singular: avstängt
• plural: avstängda

So:

• mobilen är avstängd
• huset är avstängt
• telefonerna är avstängda

Mobilen means the phone.

In Swedish, the definite form is often used where English might say my phone if the context already makes it obvious whose phone is meant.

So låta mobilen vara avstängd sounds natural, even though in English we might often say:

• leave my phone switched off

Swedish is comfortable using the definite form when the reference is clear from context.

Also, mobilen is often short for mobiltelefonen.

Because stress here is an uncountable noun, like English stress.

So Swedish says:

• mer stress = more stress

not:

• en stress in this meaning

This works like:

• mer tid = more time
• mer arbete = more work
• mer problem = more trouble/problems depending on context

You may also see mera stress, but mer stress is the more common everyday form.

Here så means so or therefore.

It connects the two ideas:

• Jag vill undvika mer stress
• så i kväll ska jag lägga mig tidigt...

So the meaning is:

• I want to avoid more stress, so tonight I’m going to go to bed early...

It introduces the result or consequence of the first idea.

The comma separates two main clauses:

• Jag vill undvika mer stress
• så i kväll ska jag lägga mig tidigt och låta mobilen vara avstängd

In Swedish, commas before coordinating words like så can be used to make the sentence easier to read, especially when the clauses are fairly long.

In short:

• the comma is natural and helpful here
• Swedish punctuation can be a little more flexible than learners expect

Without the comma, the sentence would still often be understandable, but the comma makes the structure clearer.