Se ni parolus Esperanton pli ofte, ni lernus pli rapide.

The ending -us is the Esperanto conditional mood. It usually corresponds to English would.

So:

• parolus = would speak
• lernus = would learn

In this sentence, the speaker is talking about a hypothetical situation:

• Se ni parolus Esperanton pli ofte, ni lernus pli rapide.
• If we spoke Esperanto more often, we would learn more quickly.

This is a very common pattern in Esperanto: both parts of this kind of hypothetical sentence often use -us.

That is one of the main differences between English and Esperanto.

In Esperanto, when the situation is hypothetical or unreal, it is very normal to use -us in both clauses:

• Se ni parolus..., ni lernus...

English usually does not say if we would speak in this kind of sentence, but Esperanto does use the conditional there.

So even though English has:

• If we spoke..., we would learn...

Esperanto has:

• Se ni parolus..., ni lernus...

This is standard and natural Esperanto.

Because Esperanton is the direct object of parolus.

They would speak what?

• Esperanton

In Esperanto, the direct object usually takes the accusative ending -n.

Compare:

• Ni parolas Esperanton. = We speak Esperanto.
• Ni lernas Esperanton. = We learn Esperanto.

Even though English does not change the noun here, Esperanto marks it clearly with -n.

In Esperanto, language names are usually used without an article.

So you say:

• Mi parolas Esperanton.
• Li lernas la anglan. or Li lernas anglan.

With Esperanto, using no article is completely normal here:

• parolus Esperanton

English also usually says speak Esperanto, not speak the Esperanto, so this feels fairly natural.

Pli means more, and ofte means often.

So:

• ofte = often
• pli ofte = more often

This is the regular way Esperanto forms comparatives:

• pli bona = better / more good
• pli rapide = more quickly / faster
• pli ofte = more often

There is no special word like English better or faster. Esperanto usually just uses pli + the normal adjective or adverb.

Esperanto is very regular. Instead of having many irregular comparative forms, it usually uses pli.

So:

• rapida = fast
• rapide = quickly
• pli rapide = more quickly / faster

English can say both:

• more quickly
• faster

Esperanto just uses the regular comparative pattern:

• pli rapide

The ending -e marks an adverb in Esperanto.

Adverbs describe:

• how something happens
• how often something happens
• in what way something happens

Here:

• ofte = often
• rapide = quickly

They are adverbs because they modify the verbs:

• parolus — would speak more often
• lernus — would learn more quickly

A useful comparison:

• rapida = fast (adjective, describes a noun)
• rapide = quickly (adverb, describes a verb)

Yes. Esperanto word order is fairly flexible, although the most neutral version is the one you have here:

• Se ni parolus Esperanton pli ofte, ni lernus pli rapide.

You could move some parts for emphasis, for example:

• Se ni pli ofte parolus Esperanton, ni pli rapide lernus.

That is grammatically possible, but it sounds a bit more marked or stylistic. For learners, the original order is the clearest and most natural.

Normally, no. In a full sentence like this, Esperanto usually keeps the subject in each clause:

• Se ni parolus Esperanton pli ofte, ni lernus pli rapide.

The second ni makes it clear who would learn.

Because Esperanto verbs do not change according to person the way some languages do, the subject pronoun is usually important. So keeping ni in both clauses is the normal choice.

That version uses -os, the future ending:

• parolos = will speak
• lernos = will learn

So the meaning changes.

With -us

• Se ni parolus..., ni lernus...
• hypothetical / less direct / imagined situation
• If we spoke..., we would learn...

With -os

• Se ni parolos..., ni lernos...
• future real possibility
• If we speak / if we will speak Esperanto more often, we will learn faster

So -us gives a more conditional, hypothetical sense, while -os sounds more like a real future plan or expectation.

Yes, se is the normal Esperanto word for if.

Examples:

• Se pluvas, mi restos hejme. = If it rains, I will stay home.
• Se mi havus tempon, mi venus. = If I had time, I would come.

In your sentence:

• Se ni parolus Esperanton pli ofte... = If we spoke Esperanto more often...

So se is a very common and important word to learn early.

Because Esperanto is the name of a language, and it is treated as a proper name.

So it is capitalized:

• Esperanto

This is similar to English, where language names are also capitalized:

• English
• French
• Esperanto

Not every Esperanto noun is capitalized, of course—just proper names and similar cases.

It is natural Esperanto.

Everything in it is standard and idiomatic:

• Se for if
• -us for the hypothetical conditional
• Esperanton as the direct object
• pli ofte and pli rapide as regular comparative adverbs

So this is exactly the kind of sentence you could naturally say in Esperanto.