La fakto, ke teleskopo montras krateron, interesas astronomon.

Because la fakto, ke... is a very natural way in Esperanto to package a whole statement as a noun idea: the fact that...

In this sentence, the main subject is la fakto. The clause after ke explains what that fact is.

So the structure is:

• La fakto = the fact
• ke teleskopo montras krateron = that a telescope shows a crater

Together they form the subject of interesas.

Using just ke... as the subject is much less common and would sound less smooth here.

Ke means that and introduces a content clause.

So:

• ke teleskopo montras krateron = that a telescope shows a crater

This is not a relative word like kiu. It does not mean which or who. It simply introduces the statement that defines the fact.

Because that whole ke-clause is inserted after la fakto and before the main verb interesas.

Esperanto normally separates subordinate clauses with commas. Here:

• first comma: introduces the subordinate clause
• second comma: shows where that clause ends, so the main sentence can continue

Without the second comma, it would be harder to see where the subject ends and where the main verb begins.

Esperanto has only one article: la, meaning the.

There is no separate word for a or an.

So:

• la fakto = the fact
• teleskopo = a telescope / telescope
• krateron = a crater
• astronomon = an astronomer

A bare noun in Esperanto can often be indefinite in English, depending on context.

The ending -n marks the direct object.

In teleskopo montras krateron:

• teleskopo is the subject, the thing doing the showing
• krateron is the object, the thing being shown

So krateron gets -n because it receives the action of montras.

Because teleskopo is the subject of montras, not the object.

In the clause:

• teleskopo = a telescope
• montras = shows
• krateron = a crater

This is normal subject–verb–object order. Since teleskopo is the doer of the action, it stays without -n.

Because astronomon is the direct object of interesas.

This is an important pattern:

• interesi iun = to interest someone

So:

• La fakto ... interesas astronomon = The fact ... interests an astronomer

English often prefers a different viewpoint, such as an astronomer is interested, but Esperanto here uses the active pattern X interests Y.

Yes. That is a correct rewording, but it changes the grammar and the perspective.

Original sentence:

• La fakto ... interesas astronomon
• literally: The fact ... interests an astronomer

Reworded sentence:

• Astronomo interesiĝas pri la fakto, ke...
• literally: An astronomer is interested in the fact that...

So both can express a similar idea, but:

• interesas = interests someone
• interesiĝas pri = is/becomes interested in

No, Esperanto word order is fairly flexible, especially because -n shows the object.

For example, you could also say:

Astronomon interesas la fakto, ke teleskopo montras krateron.

That still means the same thing, but it emphasizes astronomon more.

Even so, the original order is the most neutral and easiest for learners:

• subject: La fakto, ke...
• verb: interesas
• object: astronomon

Because Esperanto verbs do not change for person or number.

The ending -as always means present tense.

So:

• mi montras
• li montras
• ili montras

all use -as.

The same applies to interesas. Esperanto does not have separate forms like English interest versus interests.