Rekli su mi da pratim ozljedu i da dođem opet ako koljeno ostane natečeno.

Rekli su mi is made of three parts:

• rekli = the past participle, here masculine plural
• su = are/have auxiliary used to form the past tense
• mi = to me

So literally it is something like they have told to me, but in normal English that is just they told me.

Croatian past tense often works like this:

• rekao sam = I said
• rekla je = she said
• rekli su = they said

Because rekli is plural, the subject is plural: they.

Because Croatian uses the dative for the person who receives the information.

• mi = to me
• me = me as a direct object

With reći / govoriti in this meaning, Croatian says tell something to someone, so the person is in the dative:

• Rekli su mi = They told me
• Rekli su joj = They told her
• Rekli su nam = They told us

So mi is correct because it means to me.

Croatian often leaves out subject pronouns when they are already clear from the verb.

• Rekli su already tells you it is they
• so oni is usually unnecessary

You could say Oni su mi rekli, but that would add emphasis, like they told me as opposed to someone else.

This is very common in Croatian:

• Dođem sutra. = I’ll come tomorrow.
• Rekli su mi... = They told me...

In Croatian, after verbs like reći, tražiti, željeti, morati in many constructions, it is very common to use da + present tense instead of an English-style infinitive.

So:

• Rekli su mi da pratim ozljedu = They told me to monitor the injury
• Rekli su mi da dođem opet = They told me to come again

For an English speaker, this can feel more like:

• They told me that I should monitor...
• They told me that I should come back...

But in Croatian, this is the normal way to express this idea.

This is mostly about aspect.

• pratiti is imperfective: it describes an ongoing process or repeated monitoring
• doći is perfective: it describes one complete action, to come / to come back once

That fits the meaning perfectly:

• pratim ozljedu = keep monitoring the injury
• dođem opet = come back again (one complete return visit)

So Croatian chooses the aspect based on how the action is viewed:

• ongoing or repeated → often imperfective
• single completed event → often perfective

Because ozljedu is the accusative singular of ozljeda.

• nominative: ozljeda = injury
• accusative: ozljedu

Here it is the direct object of pratim:

• pratim što? = I monitor what?
• ozljedu

This is a very common feminine noun pattern:

• knjiga → knjigu
• rana → ranu
• ozljeda → ozljedu

Because koljeno is the subject of ostane and is therefore in the nominative singular.

In the clause:

• ako koljeno ostane natečeno

the thing doing the remaining swollen is the knee, so koljeno stays in its basic dictionary form.

Compare:

• koljeno = the knee (subject form)
• koljenu = to the knee / in the knee, depending on context
• koljena = of the knee / knees, depending on context

Here the sentence needs the nominative subject: koljeno.

Because ostane is a perfective verb form, and after ako Croatian often uses a perfective present to talk about a future possible result.

So:

• ako koljeno ostane natečeno = if the knee remains / is still swollen

This refers to a possible future situation: you monitor it, and if it still turns out to be swollen, then you come back.

If you said ostaje, that would sound more like a habitual or descriptive present:

• Koljeno ostaje natečeno nakon trčanja. = The knee stays swollen after running.

In this medical instruction, ostane is the natural choice.

Because natečeno is an adjective agreeing with koljeno.

• koljeno is neuter singular
• so the adjective must also be neuter singular

That gives:

• masculine: natečen
• feminine: natečena
• neuter: natečeno

Examples:

• prst je natečen = the finger is swollen
• ruka je natečena = the hand/arm is swollen
• koljeno je natečeno = the knee is swollen

Repeating da is very normal and often clearer in Croatian.

So:

• da pratim ozljedu i da dođem opet

makes it clear that both verbs depend on rekli su mi:

• they told me to monitor
• and they told me to come back

You may sometimes hear the second da omitted in speech, but repeating it is standard and very natural, especially in careful or formal language.

Croatian has a strong tendency for short unstressed words called clitics to appear near the beginning of the clause, usually in second position.

Here the clitics are:

• su
• mi

So the normal order is:

• Rekli su mi...

Not usually:

• Su mi rekli...

If another word comes first, the clitics usually follow that first element:

• Jučer su mi rekli... = Yesterday they told me...

So Rekli su mi is the normal neutral order.

Yes, opet is completely natural here and means again.

• dođem opet = come again / come back again

You could also say ponovno, which is a bit more formal or bookish in some contexts:

• dođem ponovno

In everyday speech, opet is extremely common and sounds very natural.

Yes, dođem is formally a present-tense form of the perfective verb doći.

Croatian perfective present often refers to a future single event, especially after words like da, ako, kad, and similar conjunctions.

So although it looks like present tense, here it means something like:

• that I come back
• in natural English: that I come back / should come back / come back

This is very normal in Croatian:

• Ako dođe, javit ću ti. = If he comes, I’ll let you know.
• Rekli su mi da dođem opet. = They told me to come back again.