U dobroj vezi važno je povjerenje, a bez poštovanja odnos brzo postane težak.

Because u here means in/within, so it requires the locative case.

• basic form: dobra veza = a good relationship
• after u: u dobroj vezi = in a good relationship

Both words change because the adjective has to agree with the noun:

• veza → vezi
• dobra → dobroj

So u dobroj vezi is the correct locative form.

Because povjerenje is a neuter singular noun, and the adjective važan has to agree with it.

The adjective changes like this:

• masculine: važan
• feminine: važna
• neuter: važno

So:

• povjerenje je važno = trust is important

Even when the word order changes, the agreement stays the same, so važno je povjerenje is still neuter because of povjerenje.

Both are correct. The difference is mainly emphasis.

• Povjerenje je važno = trust is important
• Važno je povjerenje = what is important is trust / trust is what matters

Putting važno first makes the sentence sound a bit more focused or stylistically natural in this kind of statement.

Because the preposition bez always takes the genitive case.

So:

• nominative: poštovanje = respect
• genitive: poštovanja = of respect / without respect

That is why the sentence has bez poštovanja.

They are related ideas, but they are not the same.

• povjerenje = trust
  You believe someone is reliable, honest, safe, dependable.
• poštovanje = respect
  You value someone, treat them properly, and recognize their worth.

In relationships, Croatian often talks about both because they are seen as separate qualities.

They can both be translated as relationship, but they are not used in exactly the same way.

• veza often means a romantic relationship, or a bond between two people
• odnos is broader and can mean the relationship, the dynamic, or the way two people relate to each other

So here:

• u dobroj vezi sets the context: in a good relationship
• odnos brzo postane težak focuses on the relationship as an interaction or dynamic

Using both sounds natural in Croatian.

This is about verb aspect, which is very important in Croatian.

• postane comes from postati = perfective
• postaje comes from postajati = imperfective

Here postane presents the change as a whole result: the relationship ends up becoming difficult, and that happens quickly.

So:

• brzo postane težak = quickly becomes difficult, with focus on the result
• brzo postaje težak would sound more like an ongoing process: is becoming difficult

In a general statement like this, postane is very natural because it highlights the shift into a difficult state.

Yes, težak can mean heavy, but it also very often means difficult or hard, depending on context.

Here it clearly means difficult:

• težak odnos = a difficult relationship / a hard relationship dynamic

It is also in the masculine singular form because it agrees with odnos, which is masculine singular.

It can often be translated as and, but here it has a slight contrastive feeling, something like:

• and
• while
• whereas
• sometimes almost but

So in this sentence, a links two related ideas:

• trust is important
• without respect, the relationship quickly becomes difficult

It is softer than ali, which is a stronger but.

Because Croatian does not have articles like English.

So where English says:

• in a good relationship
• the relationship

Croatian simply says:

• u dobroj vezi
• odnos

Whether something is general, specific, or definite is usually understood from context, word order, and meaning.

Usually, veza very often suggests a romantic relationship, especially in everyday speech.

So many learners will naturally understand:

• u dobroj vezi = in a good romantic relationship

But depending on context, veza can also mean a connection or link of another kind. In this sentence, though, the relationship meaning is the most natural one.