Jeg lægger mobilen fra mig, når vi spiser aftensmad.

Because at lægge means to lay / put something somewhere, while at ligge means to lie / be lying somewhere.

• Jeg lægger mobilen fra mig = I put the phone down.
• Mobilen ligger på bordet = The phone is lying on the table.

So here the speaker is doing the action of putting the phone aside, which is why Danish uses lægger.

In this sentence, lægge noget fra sig is a very common expression meaning to put something down / put something aside.

So Jeg lægger mobilen fra mig is not just word-for-word I lay the phone away from me. It is an idiomatic Danish way to say that you put it down and stop holding or using it.

The last part changes with the subject:

• jeg → fra mig
• du → fra dig
• han/hun/man → fra sig
• vi → fra os

So the sentence uses fra mig because the subject is jeg.

Mobilen is the definite form of en mobil.

• en mobil = a phone
• mobilen = the phone

In Danish, it is very normal to use the definite form when it is clear which thing you mean. Here, it naturally means the phone / my phone, depending on context.

If you say min mobil, that is also possible, but it puts more focus on ownership. The sentence as given sounds very natural and does not need min.

Når is used for when in the sense of whenever / every time or for something in the future.

Here the sentence describes a habit:

• Jeg lægger mobilen fra mig, når vi spiser aftensmad.
• I put my phone down when / whenever we eat dinner.

A learner often compares this with da, but da is usually used for a specific event in the past:

• Da vi spiste aftensmad i går, lagde jeg mobilen fra mig.
• When we ate dinner yesterday, I put my phone down.

So når is the right choice for a repeated action like this.

Because når introduces a subordinate clause, and subordinate clauses in Danish do not use normal main-clause inversion.

So you get:

• når vi spiser aftensmad
• not når spiser vi aftensmad

The order inside the subordinate clause is:

• conjunction + subject + verb

If you move that clause to the front, then the main clause shows inversion:

• Når vi spiser aftensmad, lægger jeg mobilen fra mig.

That is a very common Danish pattern.

Danish often uses the present tense for habits, routines, and general behavior.

So:

• Jeg lægger mobilen fra mig, når vi spiser aftensmad

means something like:

• This is what I do whenever we eat dinner.

English does the same thing here with the simple present: I put my phone down when we eat dinner.

So the present tense is not necessarily about right now; it can also describe a regular pattern.

Yes. Danish very often uses spise where English uses have for meals.

So:

• vi spiser aftensmad = we eat / have dinner

This is completely natural Danish. English prefers have dinner, but Danish commonly says eat dinner more directly.

Aftensmad is a compound noun:

• aften = evening
• mad = food

Danish normally writes compound nouns as one word, so aftensmad is written together.

It means dinner or the evening meal. A very literal translation would be evening food, but in normal English the natural meaning is just dinner.

Yes, absolutely. That version is also correct and very natural.

The meaning stays basically the same. The difference is mostly emphasis:

• Jeg lægger mobilen fra mig, når vi spiser aftensmad.
  Focus starts on what I do.
• Når vi spiser aftensmad, lægger jeg mobilen fra mig.
  Focus starts on the situation / time.

Notice that when the når-clause comes first, the main clause changes to lægger jeg, not jeg lægger. That is standard Danish word order after something is placed in first position.