Tio restu sekreto inter ni.

Because restu is the -u form of resti (to remain / to stay).

The -u ending is used for a command, wish, request, or exhortation. So restu means something like let it remain or may it stay.

• restas = remains / is remaining
• restos = will remain
• restu = let it remain / may it remain

So this sentence is not simply describing a fact; it is expressing what the speaker wants.

Tio means that or that thing / that matter.

It usually refers to something already mentioned, understood from context, or pointed to in some non-physical way. In a sentence like this, it often means that matter, that information, or what we just talked about.

So tio is a very common way to refer back to an idea rather than repeating it.

Because sekreto is a noun (a secret), while sekreta is an adjective (secret).

So:

• Tio restu sekreto = Let that remain a secret
• Tio restu sekreta = Let that remain secret

Both are grammatical, but they are phrased slightly differently.

The version with sekreto treats the thing as a secret.
The version with sekreta describes it with the quality secret.

English can use either style too:

• Let it remain a secret
• Let it remain secret

Because sekreto here is a predicate noun, not a specifically identified object.

After verbs like esti (to be), resti (to remain), or fariĝi (to become), Esperanto often leaves out la when the noun is being used in a general classifying sense.

So sekreto here means a secret, not the secret.

Compare:

• Tio estas problemo. = That is a problem.
• Li fariĝis kuracisto. = He became a doctor.
• Tio restu sekreto. = Let that remain a secret.

You would use la sekreto only if you meant a specific, already identified secret.

Inter ni literally means between us or among us.

• inter = between / among
• ni = we / us

In this sentence, it means something like just between us or between you and me / us.

English often uses between us in this kind of private-context sentence, and Esperanto does the same very naturally.

Because nothing here is a direct object.

Here is the structure:

• Tio = the subject
• restu = the verb
• sekreto = predicate noun
• inter ni = prepositional phrase

The accusative -n is mainly used for direct objects and for certain motion/direction uses. None of those apply here.

So:

• not sekreton, because sekreto is not a direct object
• not nin, because ni comes after the preposition inter

Yes, that is possible.

The difference is:

• estu = be
• restu = remain / stay

So:

• Tio estu sekreto inter ni = Let that be a secret between us
• Tio restu sekreto inter ni = Let that remain a secret between us

Using restu suggests continuity: maybe it is already secret, and the speaker wants it to stay that way. In this context, restu often sounds especially natural.

It can be understood as any of those, depending on tone and context.

That is normal for the -u form in Esperanto. It covers several meanings that English expresses with different structures such as:

• let ...
• may ...
• please ...
• an imperative-like command

So Tio restu sekreto inter ni could sound like:

• a gentle request
• a serious instruction
• a quiet wish

The grammar itself does not force only one of those interpretations; context does that.

Yes. Esperanto word order is fairly flexible.

The given order, Tio restu sekreto inter ni, is a very natural neutral order. But other orders are also possible for emphasis, for example:

• Inter ni, tio restu sekreto.
• Sekreto inter ni restu tio. — grammatical, but much less natural

Because Esperanto marks grammatical roles mostly with endings and prepositions rather than strict word order, speakers can move parts around more easily than in English. Still, the original version is probably the most idiomatic for ordinary use.