Ne ĵetu ŝtonon en la fonton.

The ending -u is the Esperanto verb ending for a command, request, wish, or instruction.

So:

• ĵeti = to throw
• ĵetu = throw! / should throw / let ... throw

In this sentence, Ne ĵetu ... means Don't throw ...

This is the normal way to make a negative command in Esperanto:

• Ne parolu. = Don't speak.
• Ne kuru. = Don't run.
• Ne ĵetu ŝtonon en la fonton. = Don't throw a stone into the fountain.

In Esperanto, ne normally goes directly before the thing it negates. In a simple command like this, it comes before the verb:

• Ne ĵetu = don't throw

That is the standard pattern for a negative command.

Compare:

• Ĵetu ĝin. = Throw it.
• Ne ĵetu ĝin. = Don't throw it.

The -n on ŝtonon marks the direct object.

The direct object is the thing directly affected by the action. Here, the action is throwing, and the thing being thrown is a stone.

So:

• ŝtono = a stone
• ŝtonon = a stone, as the direct object

This is very common in Esperanto:

• Mi vidas hundon. = I see a dog.
• Li aĉetis libron. = He bought a book.
• Ne ĵetu ŝtonon ... = Don't throw a stone ...

Not here. Fonton has -n for a different reason: it shows direction or movement toward a place.

With en, Esperanto distinguishes between:

• location: en la fonto = in the fountain
• movement into: en la fonton = into the fountain

So in this sentence:

• en la fonton = into the fountain

That means the sentence has:

• ŝtonon = direct object
• fonton = destination, marked with -n because of motion into it

This is an important Esperanto pattern:

• La birdo estas en la ĝardeno. = The bird is in the garden.
• La birdo flugas en la ĝardenon. = The bird flies into the garden.

The difference is:

• en la fonto = in the fountain (location, no movement)
• en la fonton = into the fountain (movement toward the inside)

Examples:

• La monero estas en la fonto. = The coin is in the fountain.
• Li ĵetis moneron en la fonton. = He threw a coin into the fountain.

English often uses different prepositions, like in versus into. Esperanto often uses the same preposition, en, and lets the -n show movement.

La means the.

So:

• la fonton = the fountain
• ŝtonon = a stone / stone, with no the

This usually means the fountain is a specific, known fountain, while the stone is not specific.

So the sentence suggests something like:

• don't throw a stone into the fountain

If you said la ŝtonon, that would mean the stone, a specific stone already known from context.

Esperanto has only one definite article, la. It has no separate word for a/an. The lack of la often gives that indefinite sense.

Yes. This is a very natural word order:

• Ne ĵetu ŝtonon en la fonton.

That is basically:

• not + verb + object + destination

Esperanto word order is fairly flexible because the endings show grammatical roles, but this order is the most straightforward and neutral.

You could rearrange things in some contexts, but it would usually sound more marked or poetic. For a learner, this sentence is an excellent model of normal word order.

These are two special Esperanto letters.

• ĵ is pronounced like the s in measure, or the g in mirage
• ŝ is pronounced like sh in shoe

So roughly:

• ĵetu sounds like ZHEH-too
• ŝtonon begins with sht
• fonton is pronounced more or less as written: FON-ton

A rough pronunciation of the whole sentence would be:

neh ZHEH-too SHTOH-non en lah fon-TON

Not exact English spelling, but close enough to help.

The dictionary form is ĵeti, which means to throw.

In Esperanto, verbs are learned from their infinitive form ending in -i:

• ĵeti = to throw
• paroli = to speak
• kuri = to run

Then the endings change depending on the use:

• ĵetas = throws / is throwing
• ĵetis = threw
• ĵetos = will throw
• ĵeti = to throw
• ĵetu = throw! / let ... throw
• ĵetus = would throw

So ĵetu is the -u form of ĵeti.

Usually in a sentence like this, yes, fonto is understood as fountain. Depending on context, it can also mean a spring or source.

So the exact English word can depend on the situation, but the grammar stays the same.

For a learner, the main thing to notice is not the vocabulary nuance, but the structure:

• ĵetu = command
• ŝtonon = direct object
• en la fonton = movement into the fountain

Yes, it could be plural, but the sentence as written uses the singular:

• ŝtonon = a stone
• ŝtonojn = stones

So:

• Ne ĵetu ŝtonon en la fonton. = Don't throw a stone into the fountain.
• Ne ĵetu ŝtonojn en la fonton. = Don't throw stones into the fountain.

The singular is perfectly natural if the warning is about even one stone. It can also feel like a general instruction.

No, not for this meaning.

ĵetas is the present-tense form, meaning throws / is throwing. So Ne ĵetas ... would mean something like does not throw ..., not don't throw ...

To give a command, Esperanto uses -u, not -as.

So:

• Li ne ĵetas ŝtonon en la fonton. = He is not throwing a stone into the fountain.
• Ne ĵetu ŝtonon en la fonton. = Don't throw a stone into the fountain.

The -u form itself does not tell you exactly how strong or polite the command is. It can express:

• a direct command
• a warning
• a request
• an instruction

So Ne ĵetu ŝtonon en la fonton. could sound like:

• a rule on a sign
• a spoken warning
• a parent telling a child what not to do

Politeness depends mostly on context, tone, and sometimes extra words, not just the -u ending itself.