Lisna uš je na bosiljku.

Yes. Lisna uš is the normal Croatian term for aphid.

Literally, it breaks down as:

• lisna = leaf / foliar
• uš = louse

So the literal sense is leaf louse, but in normal usage it simply means aphid. Croatian often uses multi-word expressions like this as fixed names for plants, animals, and insects.

Because lisna is an adjective, and it must agree with uš in:

• gender
• number
• case

Here, uš is:

• feminine
• singular
• nominative

So the adjective also has to be feminine singular nominative:

• lisna uš

Compare:

• lisni = masculine
• lisna = feminine
• lisno = neuter

Because in Croatian, not all feminine nouns end in -a.

A lot of feminine nouns do end in -a, but there is also a group of feminine nouns that end in a consonant, for example:

• noć = night
• stvar = thing
• uš = louse

So uš is simply one of those feminine nouns whose nominative singular does not have the typical -a ending.

Je is the 3rd person singular present tense of biti = to be.

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In Lisna uš je na bosiljku, je means is.

Unlike in some other Slavic languages, Croatian normally keeps the present-tense form of to be in ordinary sentences like this.

Because na can take different cases depending on the meaning.

Here the sentence describes location: the aphid is already on the basil.
With na + location, Croatian uses the locative case.

So:

• base form: bosiljak = basil
• locative singular: bosiljku

That is why the sentence has:

• na bosiljku = on the basil

If there were movement onto something, Croatian would usually use the accusative instead:

• na bosiljak = onto the basil

Because na usually means on, at, or upon, while u usually means in or inside.

An aphid sits on a plant, not inside it, so na is the natural choice:

• na bosiljku = on the basil plant

This is a very common Croatian pattern for surface location.

No. Standard Croatian does not have articles like English the or a/an.

So Lisna uš je na bosiljku can mean things like:

• An aphid is on the basil
• The aphid is on the basil

The exact definite/indefinite meaning depends on context.

That is something English speakers often have to get used to in Croatian.

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

The basic neutral order here is:

• Lisna uš je na bosiljku.

But you could also say:

• Na bosiljku je lisna uš.

That version puts more focus on the location: On the basil, there is an aphid.

Even though word order can change, you cannot rearrange words completely freely without sounding unnatural. The usual reason to change the order is emphasis or information structure.

A rough English-friendly pronunciation is:

• Lisna uš je na bosiljku ≈ LEES-na oosh ye na bo-SEELY-koo

A few useful points:

• š sounds like sh in ship
• j sounds like y in yes
• lj is a soft sound, roughly like ly in million for many learners

So:

• uš sounds like oosh
• je sounds like ye
• bosiljku has that soft lj sound in the middle

It is in the nominative singular, because it is the subject of the sentence.

The structure is:

• Lisna uš = subject
• je = verb
• na bosiljku = prepositional phrase showing location

So if you are analyzing the grammar, the sentence is essentially:

• [Nominative subject] + [to be] + [location phrase]

The plural is:

• lisne uši = aphids

So the plural sentence would be:

• Lisne uši su na bosiljku.
• The aphids are on the basil.

Notice both parts change:

• lisna → lisne
• uš → uši
• je → su

Yes, that is how it is normally understood here.

In this sentence, bosiljak is being treated as the plant the aphid is sitting on. So na bosiljku most naturally means:

• on the basil plant

That fits the real-world meaning much better than thinking of basil only as a kitchen herb or ingredient.