Bude li oporavak brz, u ponedjeljak će opet ići na nastavu.

Here bude li means if.

So Bude li oporavak brz is equivalent to Ako oporavak bude brz.

This word order is an inverted conditional structure:

• bude li = if it is / if it turns out to be
• very common in written, formal, or slightly elevated style

A natural English comparison is something like Should the recovery be quick..., although in normal translation you would usually just say If the recovery is quick...

No. Even though li is often used in yes/no questions, here the whole phrase is not a question.

In this pattern, li combines with the inverted verb to form a conditional meaning:

• Bude li... = If ... is / if ... happens to be

So this is a statement with a condition, not a question.

Because Croatian normally does not use future I in an if-clause like this.

So instead of saying something like:

• ako će oporavak biti brz — not the normal choice here

Croatian prefers:

• ako oporavak bude brz
• bude li oporavak brz

And je would sound like a present-time fact, not a future condition.

So bude is the normal form when you mean a future condition such as if the recovery is quick.

Yes, the meaning is essentially the same.

The main difference is style:

• Ako oporavak bude brz = more neutral, everyday
• Bude li oporavak brz = more formal, more written, a bit more compact

Both are correct.

There are three useful case points here:

• oporavak is nominative singular
  It is the subject of the clause.

• brz is also nominative singular masculine
  It agrees with oporavak. After biti, predicate adjectives are normally in the nominative.

• u ponedjeljak uses accusative
  This is the normal expression for on Monday.

• na nastavu also uses accusative
  After ići na, Croatian uses the accusative because this expresses movement toward an activity or destination.

So:

• oporavak = nominative
• brz = nominative
• ponedjeljak = accusative
• nastavu = accusative

Because Croatian often leaves the subject unstated when it is clear from context.

The form će ići only tells you it is third person, but it does not by itself show gender, and in this case it can also be singular or plural depending on context.

So without extra context, it could mean:

• he will go
• she will go
• they will go

Croatian does this very often, whereas English usually needs an explicit subject.

Because će is a clitic. Croatian clitics usually go in the second position in the clause.

In the main clause, the first unit is:

• u ponedjeljak

So the clitic će comes right after that:

• u ponedjeljak će opet ići na nastavu

If the clause started with a subject, you would also place će after that first element:

• On će opet ići na nastavu.

This second-position rule is one of the most important word-order patterns in Croatian.

nastava means classes, lessons, or instruction.

So ići na nastavu means:

• to go to class
• to attend classes
• to go to lessons

It does not literally focus on the school building. It focuses more on the activity of attending classes.

That is why ići na nastavu is a bit different from:

• ići u školu = to go to school

Depending on context, English may translate ići na nastavu as go back to class, go to classes, or attend lessons.

opet means again.

Here it shows that the person will resume something they were doing before. In natural English, that is often translated as:

• again
• back
• once again

So opet ići na nastavu can naturally become:

• go to class again
• go back to classes

It is perfectly correct, but the opening Bude li... sounds somewhat more formal or written than everyday speech.

In casual conversation, many speakers would be more likely to say something like:

• Ako oporavak bude brz, u ponedjeljak će opet ići na nastavu.
• Ako se brzo oporavi, u ponedjeljak će opet ići na nastavu.

So the sentence is natural, but it has a slightly more formal written feel than the most conversational version.