Hennes bästa tips är att stänga av mobilen en timme innan hon lägger sig.

Because sin/sitt/sina is a reflexive possessive, used when the possessor is the same as the subject of that clause.

Here, the sentence starts with the noun phrase Hennes bästa tips = Her best tip. The possessive is simply her, so Swedish uses hennes.

Compare:

• Hon stänger av sin mobil. = She turns off her own phone.
• Hennes bästa tips... = Her best tip...

So sin would not sound right in this sentence.

Bästa is the superlative form of bra (good), meaning best.

In Swedish, when a superlative adjective comes before a noun, it usually takes the -a form:

• en bra idé = a good idea
• en bättre idé = a better idea
• den bästa idén = the best idea

After possessives like min, din, hennes, Swedish also uses that -a form:

• hennes bästa tips = her best tip

So bästa is the correct form before the noun tips.

The form tips looks the same in both singular and plural in Swedish:

• ett tips = a tip
• flera tips = several tips

So the word form itself does not tell you.

In this sentence, it is most naturally understood as singular: her best tip. The rest of the sentence gives one specific piece of advice, so that is the intended meaning.

This is a very common Swedish structure:

• är att + infinitive

It works like English is to + verb.

So:

• Hennes bästa tips är att stänga av mobilen
  = Her best tip is to turn off the phone

Here, att is the infinitive marker, like English to.

Stänga av is a particle verb.

• stänga = to close / shut
• av = off

Together, stänga av means to turn off / switch off.

So the meaning is not just close off literally, but specifically switch off a device such as a phone, TV, or computer.

Examples:

• stänga av mobilen = turn off the phone
• stänga av datorn = turn off the computer

Because mobilen is the definite form:

• en mobil = a mobile phone / a cell phone
• mobilen = the mobile phone

Swedish usually adds the definite article as an ending on the noun.

Also, in Swedish, mobil normally means mobile phone / cell phone. So mobilen here means the phone.

In advice sentences like this, Swedish often uses the definite form where English might also say your phone depending on context.

Yes, you sometimes could, but the nuance changes a little.

• stänga av mobilen = turn off the phone
• stänga av sin mobil = turn off her/one’s own phone

Using mobilen sounds more general and natural in broad advice. Using sin mobil would put more emphasis on the fact that it is the person’s own phone.

So the original sentence sounds very idiomatic.

It means an hour before.

• en timme = an hour
• innan = before

So:

• en timme innan hon lägger sig
  = an hour before she goes to bed

It tells you the time gap between the two actions.

Because lägga sig is a reflexive verb meaning to lie down or, very commonly, to go to bed.

• lägger = present tense of lägga
• sig = reflexive pronoun, here meaning something like herself

But you should understand lägger sig as a whole expression. In this sentence, it is best translated as:

• hon lägger sig = she goes to bed

You do not translate it word-for-word as she lays herself in normal English.

This is an important difference.

• lägga sig = lie down / go to bed
  describes movement into that position
• ligga = lie / be lying
  describes the state of already being there

So:

• Hon lägger sig klockan elva. = She goes to bed at eleven.
• Hon ligger i sängen. = She is lying in bed.

In your sentence, the meaning is about the moment she goes to bed, so lägger sig is the correct choice.

Because Swedish often uses the present tense for habitual actions or general routines, even where English may also think of a future event.

This sentence is giving general advice about a routine:

• turn off the phone one hour before she goes to bed

So lägger sig is present tense because it describes what she normally does or when that action happens in relation to the advice.

Because innan introduces a subordinate clause.

In Swedish subordinate clauses, the normal word order is:

• subject + verb

So:

• innan hon lägger sig = correct

The inverted main-clause order (lägger hon sig) is not used after innan here.

Compare:

• Main clause: Nu lägger hon sig. = Now she is going to bed.
• Subordinate clause: innan hon lägger sig = before she goes to bed