Estas rusto sur parto de la stirilo.

This is an existential pattern in Esperanto, like English there is.

• Estas rusto ... = There is rust ...
• There is no separate word for English there in this kind of sentence.
• The thing that exists is rusto, so that is effectively the subject.

So Esperanto often uses:

• Estas libro sur la tablo. = There is a book on the table.
• Estas rusto sur parto de la stirilo. = There is rust on part of the steering wheel.

Esperanto has no indefinite article, so plain rusto can mean rust or some rust, depending on context.

You use la only when the noun is definite and specifically identified.

Here, rusto is being introduced as something present, not as a previously known specific rust spot, so rusto is natural.

Compare:

• Estas rusto sur la pordo. = There is rust on the door.
• La rusto sur la pordo estas malbela. = The rust on the door is ugly.

Because the sentence refers to a specific steering wheel, not just any steering wheel in general.

In context, it means the steering wheel of the vehicle being talked about. Esperanto often uses la when the listener can identify the thing from the situation.

So:

• stirilo = a steering wheel / steering device
• la stirilo = the steering wheel

It literally means part of the steering wheel.

Breakdown:

• parto = part
• de = of
• la stirilo = the steering wheel

So:

• sur parto de la stirilo = on part of the steering wheel

This is a very normal way to express possession or relationship in Esperanto: noun + de + noun.

Because Esperanto commonly expresses of relationships with de.

So parto de la stirilo works just like English part of the steering wheel.

This is often the simplest and most natural way to show that one thing belongs to, comes from, or is related to another thing.

Examples:

• la koloro de la aŭto = the color of the car
• la pordo de la domo = the door of the house
• parto de la stirilo = part of the steering wheel

Yes. Esperanto words are often built from roots and affixes.

• stir- = steer, guide, control direction
• -il- = tool or instrument
• -o = noun ending

So stirilo literally means steering tool/device.

Depending on context, that can mean steering wheel, handlebar, or another steering control, but here it clearly means steering wheel.

This is a useful pattern:

• tranĉi = to cut → tranĉilo = knife, cutting tool
• skribi = to write → skribilo = writing instrument
• stiri = to steer → stirilo = steering device

Because it is not a direct object here.

The -n ending usually marks the direct object, but in this sentence rusto is the thing that exists:

• Estas rusto ... = There is rust ...

That makes rusto the subject-like element of the existential sentence, so no -n is used.

Compare:

• Mi vidas ruston. = I see rust.
  Here ruston is a direct object, so it takes -n.

• Estas rusto sur la stirilo. = There is rust on the steering wheel.
  Here rusto is not an object, so no -n.

Yes. That is also correct.

Esperanto word order is fairly flexible, especially when the meaning is already clear.

These are both fine:

• Estas rusto sur parto de la stirilo.
• Sur parto de la stirilo estas rusto.

The first version presents the existence of rust first.
The second version puts more focus on the location.

Both mean essentially the same thing.

Sur usually means on, on top of, or on the surface of.

In this sentence, it means the rust is on the surface of part of the steering wheel.

That fits well, because rust is something that appears on a surface.

Depending on context, Esperanto uses different prepositions for different spatial ideas:

• sur = on
• en = in
• sub = under
• ĉe = at, by
• apud = beside

So sur parto de la stirilo is the natural choice for rust located on that surface.

Not exactly.

• rusto = rust, the substance or corrosion
• rustaĵo = a rusty thing, rusted matter, a rust formation

In most ordinary situations, rusto is the better and more natural word.

So if you simply want to say there is rust present, Estas rusto ... is the normal phrasing.

Not in exactly the same structure.

Havas means has, so you would need a subject:

• La stirilo havas ruston sur parto de ĝi. = The steering wheel has rust on part of it.

That is grammatical, but less natural in many contexts than the simple existential sentence:

• Estas rusto sur parto de la stirilo.

Esperanto often prefers the existential form when you are just pointing out that something is present somewhere.

Only one area.

The phrase sur parto de la stirilo specifically means on part of the steering wheel. It tells you the rust is not necessarily everywhere.

If you wanted to say the rust is on the whole steering wheel, you would use a different expression, such as:

• Estas rusto sur la stirilo. = There is rust on the steering wheel.

Adding parto makes the location more limited and specific.