Estas ŝimo sur la poto.

Because this is an existential sentence: it means there is / there are.

So Estas ŝimo sur la poto literally works like There is mold on the pot.

In Esperanto, you often use just estas for this kind of idea, without a separate dummy subject like English there.

• Estas ŝimo sur la poto. = There is mold on the pot.
• Estas libro sur la tablo. = There is a book on the table.

You would not normally say Ĝi estas ŝimo sur la poto for this meaning.

Because ŝimo is not the direct object of an action verb here. It is simply the thing that exists.

The verb esti means to be, and nouns after esti do not take the accusative -n just because they come after the verb.

So:

• Estas ŝimo... = There is mold...
• not Estas ŝimon...

The -n ending is mainly used for:

• direct objects
• direction toward somewhere in some cases

Neither applies to ŝimo here.

Because the sentence is talking about some mold or mold in general, not a specific previously identified mold.

Compare:

• Estas ŝimo sur la poto. = There is mold on the pot.
• La ŝimo sur la poto estas verda. = The mold on the pot is green.

In the first sentence, the mold is being introduced for the first time, so no article is needed.

Sur means on or on top of.

So:

• sur la poto = on the pot

This tells you the location of the mold.

It is used because the mold is located on the surface of the pot.

Because la means the, and here the pot is being treated as a specific, identifiable pot.

So:

• la poto = the pot
• poto = a pot / pot in general

In many contexts, Esperanto uses la much like English uses the: when the listener is expected to know which thing is meant.

Poto means pot.

Like most Esperanto nouns, it ends in -o:

• poto = pot
• ŝimo = mold

The ending -o is the standard noun ending in Esperanto.

Yes. Esperanto word order is fairly flexible, as long as the meaning stays clear.

For example, you could also say:

• Sur la poto estas ŝimo.

That still means There is mold on the pot.

The difference is mostly one of emphasis:

• Estas ŝimo sur la poto. focuses first on the existence of mold.
• Sur la poto estas ŝimo. focuses first on the location.

Both are natural.

Ŝ is pronounced like English sh.

So ŝimo is pronounced roughly like SHEE-mo.

Breakdown:

• ŝi sounds like shee
• mo sounds like mo

Also, Esperanto stress normally falls on the second-to-last syllable, so:

• ŜI-mo

In Esperanto, the verb esti changes by tense, but not by person or number.

So estas can mean:

• am
• is
• are

depending on context.

That means you use estas for both singular and plural existential sentences:

• Estas ŝimo sur la poto. = There is mold on the pot.
• Estas makuloj sur la poto. = There are stains on the pot.

So the verb form stays estas either way.

Possibly, depending on context.

Ŝimo is a general word for mold and can cover what English speakers might call mold or sometimes mildew, depending on the situation.

If the exact scientific distinction matters, a more specific term might be needed, but in ordinary usage ŝimo is the natural everyday word.

The difference is:

• sur la poto = on the pot
• en la poto = in the pot

So:

• Estas ŝimo sur la poto. means the mold is on the outside surface or top of the pot.
• Estas ŝimo en la poto. means the mold is inside the pot.

That preposition changes the meaning clearly.

You usually would not say that unless you mean something more general or non-specific in a special context.

• sur la poto = on the pot, a specific pot
• sur poto = on a pot, or pot-like surface, which sounds less natural here

In a normal sentence about a particular object, sur la poto is the expected form.

Yes. It is a fully complete and natural sentence.

It has:

• the verb: estas
• the thing that exists: ŝimo
• the location: sur la poto

So grammatically and naturally, it stands on its own just fine.