Hun sætter også sin mobil på lydløs, før forelæsningen begynder.

Because sin/sit/sine is the reflexive possessive in Danish. It is used when the thing belongs to the subject of the clause.

In Hun sætter også sin mobil på lydløs, the subject is hun, so sin means her own.

• Hun sætter sin mobil på lydløs = She puts her own phone on silent.
• Hun sætter hendes mobil på lydløs = She puts someone else’s phone on silent.

So sin is the natural choice here.

The form depends on the gender/number of the noun being possessed.

The noun is mobil, and en mobil is a common-gender singular noun, so the correct form is sin.

The reflexive possessive forms are:

• sin for en-words singular
• sit for et-words singular
• sine for plural nouns

Examples:

• sin mobil
• sit bord
• sine bøger

Literally, it is something like puts ... on silent.

This is a very natural Danish way to say that a phone is being switched to silent mode:

• sætte mobilen på lydløs = put the phone on silent / set the phone to silent

So på lydløs works like an idiomatic expression here.

In Danish, with verbs like sætte, it is common to use på to show the state or mode something is put into.

So:

• sætte noget på lydløs = set something to silent
• sætte noget på pause = put something on pause

You usually learn på lydløs as a fixed expression.

Også means also or too.

Its position is normal Danish word order. In a main clause, Danish usually has the finite verb in second position, and adverbs like også often come after that verb:

• Hun sætter også sin mobil på lydløs.

Compare English:

• She also puts her phone on silent.
• She puts her phone on silent too.

In Danish, også can move depending on what is being emphasized, but this placement is very natural.

Because før introduces a subordinate clause.

In Danish subordinate clauses, the word order is usually more straightforward: subject before verb.

So:

• før forelæsningen begynder = before the lecture begins

Here:

• forelæsningen = the lecture
• begynder = begins

This is normal subordinate-clause word order in Danish.

Forelæsning means lecture.
Forelæsningen means the lecture.

The ending -en is the usual definite article ending for many common-gender nouns in Danish:

• en forelæsning = a lecture
• forelæsningen = the lecture

Unlike English, Danish often puts definiteness at the end of the noun instead of using a separate word like the.

Begynder is the present tense of begynde = to begin.

So:

• at begynde = to begin
• begynder = begins / is beginning

In this sentence, it means:

• før forelæsningen begynder = before the lecture begins

Sætter is the present tense of sætte = to put / set / place.

So:

• at sætte = to put / set
• sætter = puts / sets

In this sentence:

• Hun sætter også sin mobil på lydløs = She also puts/sets her phone on silent

Yes. Mobil is a very common everyday short form for mobile phone.

You may also see:

• mobiltelefon = mobile phone

But in normal speech, mobil is extremely common and natural.

No. In this sentence, før means before in time.

For physical position, Danish usually uses foran:

• før forelæsningen begynder = before the lecture begins
• foran bygningen = in front of the building

So før is about time here, not location.

Because sin mobil already means her phone or more literally her own phone. The possessive takes the place of an article.

Compare:

• mobilen = the phone
• sin mobil = her own phone
• hendes mobil = her phone / someone else’s phone, depending on context

So you do not need a separate word meaning the here.

Yes, in natural English you could say several things:

• She also puts her phone on silent before the lecture begins.
• She also sets her phone to silent before the lecture begins.
• She also turns her phone to silent before the lecture begins.
• She also silences her phone before the lecture begins.

The Danish structure is closest to puts/sets ... on/to silent, but the exact English wording can vary.