Det afhænger af vejret, om vi spiser aftensmad i haven.

In this sentence, det is a formal subject or dummy subject. Danish often uses det at the beginning of a clause when the real content comes later.

So in:

Det afhænger af vejret, om vi spiser aftensmad i haven.

the real thing that depends on the weather is the whole clause:

om vi spiser aftensmad i haven

A more literal way to think of it is:

It depends on the weather whether we eat dinner in the garden.

English does something very similar with it.

At afhænge af is a fixed verb expression meaning to depend on.

• afhænger = depends
• af = on / on the basis of

So:

• Det afhænger af vejret = It depends on the weather

You should learn at afhænge af as a unit, because the preposition af belongs naturally with the verb.

Vejr means weather, and vejret means the weather.

The ending -et is the definite article for many neuter nouns in Danish. Instead of putting a separate word like the before the noun, Danish usually adds the definite ending to the noun itself.

• vejr = weather
• vejret = the weather

Since English says the weather, Danish says vejret.

Here, om means whether.

So:

om vi spiser aftensmad i haven = whether we eat dinner in the garden

This is not the same as om meaning about or in/around in other contexts. Danish om has several uses, and this is one of the most common ones.

Because om vi spiser aftensmad i haven is a subordinate clause.

In Danish subordinate clauses, the word order is usually more like English:

• vi spiser = we eat

You do not invert the verb and subject after om.

So:

• correct: om vi spiser
• not correct: om spiser vi

Compare:

• main clause question: Spiser vi aftensmad i haven? = Are we eating dinner in the garden?
• subordinate clause: ... om vi spiser aftensmad i haven = ... whether we are eating dinner in the garden

Because Danish punctuation normally places a comma before a subordinate clause.

So the comma separates:

• Det afhænger af vejret
• om vi spiser aftensmad i haven

This is very standard in written Danish. English punctuation is often lighter here, but Danish uses commas more systematically before subordinate clauses.

Because i haven means in the garden, and that is the normal Danish expression.

• i = in
• haven = the garden

So:

• i haven = in the garden

For locations, Danish prepositions do not always match English one-to-one, but in this case i is the natural choice.

Aftensmad literally means evening food:

• aften = evening
• mad = food

In everyday usage, it usually means dinner / supper / evening meal.

So:

• spiser aftensmad = eat dinner / have dinner

Sometimes, yes, but it depends on region and habit.

In modern standard Danish, aftensmad is a very common and clear word for the evening meal.
Middag can also refer to the main meal, but in some contexts it may sound more traditional, regional, or can cause confusion because English midday suggests lunch.

For a learner, aftensmad is the safest and most transparent choice for dinner in the evening.

Yes. Danish allows some rephrasing. For example:

Om vi spiser aftensmad i haven, afhænger af vejret.

This means the same thing:
Whether we eat dinner in the garden depends on the weather.

The original version with det is very natural and common:

Det afhænger af vejret, om vi spiser aftensmad i haven.

Both are correct, but the original may sound slightly smoother and more idiomatic in everyday Danish.

Yes. Spiser is present tense of at spise = to eat.

But just like in English, present tense can be used for a future situation when the context makes it clear.

So here:

om vi spiser aftensmad i haven

literally looks like whether we eat dinner in the garden, but in context it can naturally mean:

whether we will eat dinner in the garden

because the decision depends on the weather.

The sentence has this structure:

Det + afhænger af vejret + om-clause

Broken down:

• Det = formal subject
• afhænger = depends
• af vejret = on the weather
• om vi spiser aftensmad i haven = whether we eat dinner in the garden

So the pattern is:

Det afhænger af X, om Y = It depends on X whether Y

This is a very useful sentence pattern in Danish.

A native speaker would understand it as: the weather is the deciding factor.

The sentence does not mainly focus on the act of eating dinner. It focuses on the condition behind the decision.

So the communicative meaning is:

We might eat dinner in the garden, but that depends on the weather.

That is why Det afhænger af vejret comes first: it sets up the condition before giving the specific thing that depends on it.