Nivo gvožđa i zdravstveno stanje su dobri.

Because the subject is actually two things joined by i:

• nivo gvožđa = iron level
• zdravstveno stanje = health condition/status

Together, they form a compound subject, so Serbian uses the plural of biti:

• je = is
• su = are

So:

• Nivo gvožđa je dobar. = The iron level is good.
• Zdravstveno stanje je dobro. = The health condition is good.
• Nivo gvožđa i zdravstveno stanje su dobri. = The iron level and health condition are good.

Because dobri agrees with the whole compound subject, not just with one noun.

The two nouns are:

• nivo — masculine singular
• stanje — neuter singular

When Serbian has a mixed-gender compound subject, the predicate adjective usually goes into the plural, and masculine plural is the default form here:

• dobar = masculine singular
• dobro = neuter singular
• dobri = masculine plural

So:

• nivo je dobar
• stanje je dobro
• nivo i stanje su dobri

Gvožđa is in the genitive singular.

The basic noun is:

• gvožđe = iron

After nouns like nivo meaning level, Serbian commonly uses the genitive to show of what:

• nivo gvožđa = level of iron
• literally: level of iron

So gvožđe changes to gvožđa because it depends on nivo.

This is very common in Serbian:

• nivo šećera = sugar level
• nivo vitamina D = vitamin D level
• količina vode = amount of water

It is a normal noun phrase:

• stanje = condition, state
• zdravstveno = health-related, medical

So zdravstveno stanje means health condition, state of health, or health status.

Grammatically:

• zdravstveno is an adjective
• stanje is the noun it modifies

They agree with each other:

• zdravstveno = neuter singular
• stanje = neuter singular

It may look unusual at first, but here zdravstveno is an adjective, not an adverb.

The noun stanje is neuter singular, so the adjective must also be neuter singular:

• masculine: zdravstveni
• feminine: zdravstvena
• neuter: zdravstveno

So:

• zdravstveno stanje = health condition

Compare:

• zdravstveni problem = health problem
• zdravstvena zaštita = healthcare
• zdravstveno stanje = health condition

Because Serbian does not have articles like the or a/an.

English says:

• the iron level
• the health condition

Serbian simply says:

• nivo gvožđa
• zdravstveno stanje

Whether something means a, the, or something more general is understood from context.

Yes. Serbian word order is more flexible than English word order.

The neutral, natural version is:

• Nivo gvožđa i zdravstveno stanje su dobri.

But depending on context or emphasis, you might also hear variations like:

• Dobri su nivo gvožđa i zdravstveno stanje.
• Zdravstveno stanje i nivo gvožđa su dobri.

That said, the original sentence is the most straightforward and natural for a learner.

Yes, that is the literal structure.

• nivo = level
• gvožđa = of iron

So literally it is level of iron.

In more natural English, you would usually say iron level. Serbian often uses this noun + genitive structure where English prefers a noun used like an adjective.

In Serbian, gvožđe is the standard everyday word for iron.

A learner may also encounter regional pronunciation differences:

• gvožđe — Ekavian
• željezo — more common in some other standard varieties in the region

If you are learning standard Serbian as commonly spoken in Serbia, gvožđe is completely normal.

Because Serbian, like English, normally uses one shared verb and one shared predicate when both parts of the subject are being described together.

So instead of saying something like:

• Nivo gvožđa je dobar i zdravstveno stanje je dobro

the sentence combines them:

• Nivo gvožđa i zdravstveno stanje su dobri.

This is more compact and natural when the same idea applies to both.

In standard grammar, dobri is the better and expected form here.

Because the subject contains nouns of different genders, Serbian normally uses masculine plural as the default agreement form:

• nivo — masculine
• stanje — neuter
• therefore: su dobri

Using dobra here would sound wrong to most learners following standard rules.

The dictionary forms are:

• nivo = level
• gvožđe = iron
• zdravstven is the adjective stem, and the form used here is zdravstveno
• stanje = condition, state
• biti = to be
• dobar = good

In the sentence, some words appear in forms required by grammar:

• gvožđa = genitive singular of gvožđe
• su = 3rd person plural of biti
• dobri = masculine plural of dobar

You would match the verb and adjective to that one noun.

For nivo:

• Nivo gvožđa je dobar.
  The iron level is good.

For stanje:

• Zdravstveno stanje je dobro.
  The health condition is good.

This is a useful way to see why the combined sentence changes to su dobri:

• je dobar
• je dobro
• together: su dobri