Ako vidite lisnu uš, operite bosiljak.

Ako means if.

After ako, Croatian normally uses the present tense for a real or possible condition:

• Ako vidite lisnu uš... = If you see an aphid...

This works much like natural English If you see..., not If you will see.... So the present tense here is completely normal.

Here it means you see.

The sentence has two parts:

• Ako vidite lisnu uš = the condition
• operite bosiljak = the command

So the actual instruction is operite. Even though vidite can be an imperative form in other contexts, here the word ako makes it clear that it is part of an if-clause, not a command.

Both forms are 2nd person plural.

In Croatian, that can mean either:

• you all
• you singular, but politely/formally

So this sentence could be addressed to several people, or to one person in a polite way.

If you were speaking to one friend informally, you would usually say:

• Ako vidiš lisnu uš, operi bosiljak.

Croatian often leaves subject pronouns out because the verb ending already tells you who the subject is.

Here, the ending -te in vidite and operite already shows you plural or polite singular. Because of that, adding vi is usually unnecessary unless you want emphasis or contrast.

Operite comes from the perfective verb oprati, while perite comes from the imperfective verb prati.

That difference is about aspect:

• operite suggests wash it once / do it as a complete action
• perite suggests keep washing / wash repeatedly / wash in a general ongoing sense

In this sentence, the idea is a single response to a situation, so operite is the natural choice.

Also, the verb shows a stem change:

• oprati
• operi
• operite

So even though the dictionary form is oprati, the imperative is operi / operite.

Because it is the direct object of vidite, so it is in the accusative singular.

The basic dictionary form is:

• lisna uš = nominative singular

In this sentence it becomes:

• lisnu uš = accusative singular

The adjective changes from lisna to lisnu to match the case.

The noun uš itself does not change here. That is normal for this type of feminine noun: its nominative and accusative singular look the same.

It is also in the accusative singular, because it is the object of operite.

But bosiljak is a masculine inanimate noun, and masculine inanimate nouns usually have the same form in the nominative and accusative singular:

• nominative: bosiljak
• accusative: bosiljak

So the case changes grammatically, but the visible form stays the same.

Because Croatian does not have articles like English a / an / the.

Croatian speakers understand whether something is definite or indefinite from the context. So lisnu uš can mean an aphid or the aphid, depending on the situation, and bosiljak can mean basil or the basil.

That is completely normal in Croatian.

Yes, the word order can change.

The given version is very natural:

• Ako vidite lisnu uš, operite bosiljak.

Because the if-clause comes first, it is followed by a comma.

You can also put the main clause first:

• Operite bosiljak ako vidite lisnu uš.

That is also natural. Croatian word order is more flexible than English, although changing it can slightly change emphasis.