Se vi akceptos tiun regulon, neniu devos rifuzi la ideon poste.

Esperanto is different from English here.

In Esperanto, after se (if), you can use the tense that matches the actual time of the action. Since the accepting is in the future, akceptos is completely normal:

• Se vi akceptos... = If you accept / if you will accept...

English usually avoids will in this kind of if-clause, but Esperanto does not have that restriction.

So this sentence is natural Esperanto:

• Se vi akceptos tiun regulon...

It simply means that the accepting is expected to happen in the future.

Because the obligation is also in the future.

• devi = to have to, must
• devos = will have to

So:

• neniu devos rifuzi... = no one will have to reject...

The sentence describes a future situation:

1. first, someone accepts the rule;
2. then later, no one will need to reject the idea.

Both events are future from the speaker's point of view, so akceptos and devos both make sense.

They are both direct objects, so they take the accusative ending -n.

Breakdown:

• tiu regulo = that rule / this rule

• tiun regulon = that rule / this rule, as the object of the verb

• la ideo = the idea
• la ideon = the idea, as the object of the verb

In the sentence:

• akceptos tiun regulon = will accept that rule
• rifuzi la ideon = to reject the idea

The verbs akcepti and rifuzi both take direct objects, so the nouns get -n.

Because neniu is the subject here, not the object.

• neniu = no one, nobody
• neniun = no one, nobody, as a direct object

In this sentence:

• neniu devos rifuzi la ideon poste

Here neniu is the one doing the action of devos rifuzi, so it is the subject. Subjects do not take -n.

Compare:

• Neniu venis. = No one came.
• Mi vidis neniun. = I saw no one.

Here neniu is a pronoun meaning no one or nobody.

It belongs to the correlative system in Esperanto:

• iu = someone
• ĉiu = everyone
• neniu = no one

It can also act like an adjective before a noun:

• neniu homo = no person
• neniu ideo = no idea

But in your sentence it stands alone, so it is a pronoun:

• neniu devos... = no one will have to...

Yes. Rifuzi means to refuse, to reject, and it normally takes a direct object directly.

Examples:

• Mi rifuzis la proponon. = I rejected the proposal.
• Ŝi rifuzos la oferton. = She will refuse the offer.

So:

• rifuzi la ideon = to reject the idea

This is straightforward Esperanto grammar: the thing being rejected is the direct object, so it gets -n.

Poste means afterward, later, or subsequently.

In this sentence:

• neniu devos rifuzi la ideon poste
• no one will have to reject the idea later

It is placed at the end because Esperanto word order is fairly flexible, and adverbs like poste can often move around without changing the core meaning much.

For example, these are all possible:

• ...rifuzi la ideon poste
• ...poste rifuzi la ideon
• ...rifuzi poste la ideon

The version in your sentence is natural and clear.

Tiu usually means that, but in practice it can sometimes correspond to English this depending on context.

So:

• tiu regulo = that rule
• tiun regulon = that rule, as object

Esperanto has a separate word ĉi tiu for this in the more explicit, near sense:

• ĉi tiu regulo = this rule
• ĉi tiun regulon = this rule, as object

If your translation says this rule, that may come from the context. But literally, tiun regulon by itself is usually understood as that rule.

Because the speaker treats the two nouns differently.

• tiun regulon already has a determiner: tiun = that
• la ideon has the definite article la = the

In Esperanto, you normally do not combine la with tiu:

• tiu regulo = that rule
• not usually la tiu regulo

So each noun is already definite, just in different ways:

• tiun regulon = that specific rule
• la ideon = the specific idea

Yes, but the meaning would change.

Your sentence uses the future:

• Se vi akceptos tiun regulon, neniu devos rifuzi la ideon poste.
• This presents a real future possibility: If you accept that rule, no one will have to reject the idea later.

If you said:

• Se vi akceptus tiun regulon, neniu devus rifuzi la ideon poste.

that would sound more hypothetical:

• If you were to accept that rule, no one would have to reject the idea later.

So:

• -os = future, more direct/real future situation
• -us = conditional, more hypothetical

Not very strictly. Esperanto word order is fairly flexible because the accusative -n helps show what is the object.

Original:

• Se vi akceptos tiun regulon, neniu devos rifuzi la ideon poste.

Possible variations:

• Se vi akceptos tiun regulon, poste neniu devos rifuzi la ideon.
• Se tiun regulon vi akceptos, neniu devos poste rifuzi la ideon.

These can all work, though the original sounds natural and neutral.

The most important thing is that:

• vi and neniu are subjects
• tiun regulon and la ideon are objects because of -n

So the endings carry a lot of the grammatical information, not just the position in the sentence.