Det er vigtigt at være ærlig over for sine venner.

Here det is a dummy subject or formal subject, just like it in English sentences such as It is important to be honest.

The real idea comes in the infinitive phrase at være ærlig over for sine venner. So the structure is:

• Det er vigtigt ... = It is important ...
• at være ærlig over for sine venner = to be honest with one’s friends

Danish very often uses det er + adjective + at... in this kind of sentence.

Because the adjective vigtig changes to vigtigt when it is linked to det or to a whole clause/idea.

Compare:

• Bogen er vigtig = The book is important
  
  • bog is common gender, so vigtig
• Det er vigtigt at læse = It is important to read
  
  • det / the whole action is treated as neuter, so vigtigt

So in Det er vigtigt at være ærlig..., the -t form is required.

At is the infinitive marker, like English to, and være means be.

So:

• at være = to be

After det er vigtigt, Danish commonly uses an infinitive phrase:

• Det er vigtigt at forstå det. = It is important to understand it.
• Det er vigtigt at være ærlig. = It is important to be honest.

Because ærlig is describing the understood person who is being honest, not the word det.

In other words:

• vigtigt agrees with det / the whole idea
• ærlig describes the person implied by at være = to be

The sentence means something like for a person to be honest with their friends. Since the understood subject is a person in a general sense, Danish uses the basic form ærlig.

You would use ærligt if the thing being described were a neuter singular noun, for example:

• Barnet er ærligt. = The child is honest.

But here it is a general human subject, so ærlig is natural.

In this sentence, over for means something like toward, with, or in relation to.

So:

• ærlig over for sine venner = honest with one’s friends

This is a very common combination in Danish:

• være ærlig over for nogen = to be honest with someone

So it is best to learn ærlig over for as a set expression.

Because Danish uses the reflexive possessive sin/sit/sine when the owner is the same as the subject of the clause.

Here, the understood subject is the same person whose friends are meant:

• at være ærlig over for sine venner = to be honest with one’s own friends

If you used hans, hendes, or deres, it would usually point to someone else’s friends, not the subject’s own friends.

So this sentence is making a general statement about a person being honest with their own friends.

Because sin/sit/sine must agree with the thing owned, not with the owner.

Here the possessed noun is venner = friends, which is plural, so the correct form is sine.

The pattern is:

• sin
  • common gender singular noun
• sit
  • neuter singular noun
• sine
  • plural noun

Examples:

• sin ven = his/her own friend
• sit barn = his/her own child
• sine venner = his/her own friends

This version is general. It means something like:

• It is important to be honest with one’s friends
• It is important to be honest with your friends in a general sense

It is not specifically addressing one person directly.

If you wanted to speak directly to someone, you might say:

• Det er vigtigt at være ærlig over for dine venner.

That sounds more like It’s important to be honest with your friends said to you specifically.

Yes, that is grammatically correct.

It means the same thing, but the style is a little different:

• Det er vigtigt at være ærlig over for sine venner is the more natural and common everyday pattern
• At være ærlig over for sine venner er vigtigt is a bit more formal or written in tone

Danish often prefers the version with det er ... at ..., just as English often prefers It is important to... over To be honest with one’s friends is important.