Jeg lægger hovedtelefonen ved siden af computeren, før mødet begynder.

Because Danish distinguishes between:

• lægge = to lay / put / place something somewhere
• ligge = to lie / be lying somewhere

In this sentence, jeg is actively placing the headphones, so Danish uses lægger:

• Jeg lægger hovedtelefonen ved siden af computeren. = I put the headphones next to the computer.

If the headphones were already there, you would use ligger:

• Hovedtelefonen ligger ved siden af computeren. = The headphones are lying next to the computer.

This is the same basic contrast as lay vs lie in English, although English speakers often do not keep that distinction very clearly in everyday speech.

That is a very natural question.

In English, headphones is almost always plural. In Danish, many speakers would also naturally use the plural hovedtelefonerne if they mean a normal pair of headphones.

However, hovedtelefonen can still appear if the speaker treats it as one device/unit, or if the context suggests a single earpiece.

So:

• hovedtelefonen = singular definite
• hovedtelefonerne = plural definite

If you mean an ordinary pair of headphones, many learners will hear hovedtelefonerne more often in real life. So the sentence is possible, but the plural may sound more idiomatic in some contexts.

Because the sentence is talking about specific, identifiable things:

• the headphones
• the computer
• the meeting

Danish often uses the definite form when the speaker and listener are expected to know which thing is meant.

Compare:

• Jeg lægger en hovedtelefon ved siden af en computer.
  = I put a headphone next to a computer.
  (new, non-specific information)

• Jeg lægger hovedtelefonen ved siden af computeren, før mødet begynder.
  = I put the headphone(s) next to the computer before the meeting begins.
  (specific, known items)

In Danish, the is usually attached to the end of the noun instead of standing as a separate word.

Examples from the sentence:

• en hovedtelefon → hovedtelefonen
• en computer → computeren
• et møde → mødet

So:

• -en is a common definite ending for common gender nouns
• -et is a common definite ending for neuter nouns

That means you need to know the noun’s gender:

• en computer → computeren
• et møde → mødet

English speakers often find this unusual at first, because English puts the before the noun instead.

Ved siden af is a fixed expression meaning:

• beside
• next to

So:

• ved siden af computeren = next to the computer

Literally, it is something like at the side of, but you should learn it as one whole phrase.

A useful thing to remember is that the af is part of the expression when you name what something is beside:

• ved siden af computeren
• ved siden af døren
• ved siden af mig

Because Danish, like English, often uses the present tense for future events, especially when they are planned, expected, or scheduled.

So:

• før mødet begynder
  literally: before the meeting begins

Even if the meeting has not started yet, present tense is completely normal here.

English does the same thing in many cases:

• before the meeting begins
• when the train arrives
• after class ends

So this part of the Danish sentence is actually very close to English grammar.

The comma is acceptable, but it is not always required.

In modern Danish, both of these can be correct:

• Jeg lægger hovedtelefonen ved siden af computeren, før mødet begynder.
• Jeg lægger hovedtelefonen ved siden af computeren før mødet begynder.

This depends on the comma system being used. Danish allows both a style with start comma and one without start comma before subordinate clauses.

So if you see the comma there, do not worry: it is normal Danish punctuation.

Yes, absolutely:

• Før mødet begynder, lægger jeg hovedtelefonen ved siden af computeren.

This is a very useful example of Danish word order.

When something other than the subject comes first in a main clause, Danish usually puts the verb before the subject. This is the famous V2 pattern.

So:

• Jeg lægger ...
  subject first

but

• Før mødet begynder, lægger jeg ...
  time clause first, so the verb lægger comes before jeg

That word order is very important in Danish.

A few parts are especially difficult for English speakers:

• jeg is not pronounced with a hard English g. In careful speech it is roughly like yai, and in everyday speech it is often reduced even more.
• æ in lægger is somewhat like the vowel in cat, but not exactly the same.
• ø in før and mødet has no exact English equivalent. A rough starting point is the vowel in British English bird, but with rounded lips.

So a learner-friendly approximation might be:

• Jeg ≈ yai
• lægger ≈ LEH-ger with a Danish æ
• før ≈ a rounded fur, but not the English u
• mødet ≈ MØ-eth / MØ-thed depending accent and level of precision

The best way to improve this is to listen to native audio, because Danish pronunciation is often less straightforward than the spelling suggests.