Между друзьями важно доверие.

Because между requires the instrumental case.

• друзья = nominative plural, friends
• друзей = genitive/accusative plural
• друзьями = instrumental plural

So:

• между друзьями = between / among friends

This is a very common pattern:

• между домами = between the houses
• между людьми = among people

Literally, it means between friends or among friends.

In this sentence, it means something like:

• in friendships
• in relations between friends
• among friends

It is not only about physical position. It describes the relationship or social space where trust matters.

In Russian, между can be used with a plural noun and cover both ideas.

So между друзьями can mean:

• between friends
• among friends

English often distinguishes between and among, but Russian is less strict here. The sentence sounds general: trust is important in friendship / in friendships.

Доверие is in the nominative case.

Here it is the thing being talked about: trust. In English, you can think of it as the subject of the sentence:

• Trust is important between friends.

So the basic structure is:

• доверие = nominative noun
• важно = predicate, important

Because доверие is usually an abstract noun, like trust in English.

Abstract nouns are often singular when talking about the general idea:

• любовь = love
• счастье = happiness
• доверие = trust

So доверие here means trust as a general quality, not separate individual instances of trust.

Because важно is the short-form predicate used to mean is important.

Compare:

• важный = full adjective form, usually used before a noun
  
  • важный вопрос = an important question
• важно = short-form predicate / predicative form
  
  • это важно = this is important

Here the sentence is predicative, so Russian uses важно.

Also, доверие is neuter singular, and the neuter singular short form is важно.

You can compare:

• поддержка важна = support is important
• советы важны = advice/tips are important
• доверие важно = trust is important

Because Russian usually omits the verb "to be" in the present tense.

So instead of saying something like Trust is important, Russian simply says:

• Доверие важно.

The idea of is is understood automatically.

This is one of the most basic features of Russian sentence structure.

Yes, it is normal. Russian word order is flexible, and changing the order often changes emphasis, not the core meaning.

Starting with Между друзьями puts the relationship context first:

• As for friends / In friendship, trust is important.

So the sentence highlights the setting first, then names what matters.

Other possible orders are:

• Доверие важно между друзьями.
• Важно доверие между друзьями.

These are understandable too, but they feel slightly different in emphasis. The original sentence sounds like it is framing the topic first: between friends...

Yes, and that would be very natural.

The difference is:

• Между друзьями важно доверие.
  Focuses on trust as a quality or principle.

• Между друзьями важно доверять друг другу.
  Focuses on the action: to trust each other.

So the first is more noun-based and a little more abstract. The second is more verbal and practical.

By itself, it usually sounds general:

• among friends
• in friendship
• between friends in general

Russian has no articles like a, an, or the, so context decides whether something is general or specific.

Without any extra context, a learner should understand this as a general statement about friendship.

That is a very good way to feel the structure.

Because of the word order, the sentence can give a slight nuance like:

• Between friends, what matters is trust.
• Among friends, trust is important.

So yes, it is not just a plain factual statement. It can also sound a little emphatic, as if identifying the key value in friendship.

Both ways can help, depending on what is easier for you.

You can think of the whole sentence as:

• Trust is important between friends.

But if you are analyzing the Russian structure, it can also help to feel важно as a predicate meaning:

• is important
• it is important

In actual usage, Russian learners often just memorize that важно is the form used in sentences like:

• Это важно.
• Нам важно понять.
• Доверие важно.

So in this sentence, важно is doing the job of the English predicate is important.