Tra la teleskopo ni vidas krateron.

Because krateron is the direct object of vidas.

• vidi = to see
• ni vidas = we see
• what do we see? krateron = a crater

In Esperanto, direct objects normally take the accusative ending -n.

• kratero = a crater
• krateron = a crater (as the thing being seen)

So:

• Ni vidas kratero would be incorrect.
• Ni vidas krateron is correct.

Esperanto has only one article: la, meaning the.

So:

• la teleskopo = the telescope
• krateron = a crater / crater as an indefinite object

Esperanto does not have a separate word for a/an. If there is no la, the noun is usually understood as indefinite or general.

So this sentence contrasts:

• la teleskopo = a specific telescope, probably known from context
• krateron = some crater, not identified as a particular one

Tra usually means through.

In this sentence, Tra la teleskopo means through the telescope—that is, by looking through it.

This is quite natural, because a telescope is literally something you look through.

A learner may compare this with English:

• through the telescope = looking through the instrument
• with the telescope = using the instrument

In Esperanto, tra emphasizes the idea of looking through the telescope itself.

Yes, that is possible, but the nuance is a little different.

• tra la teleskopo = through the telescope, focusing on the physical act of looking through it
• per la teleskopo = by means of the telescope, focusing more on the telescope as an instrument

In many contexts, both could make sense.
For a telescope, tra often feels especially natural because you literally see something by looking through it.

Both are possible.

Esperanto word order is fairly flexible, because endings like -n help show grammatical roles. The version you have starts with Tra la teleskopo to give that part emphasis or to set the scene first.

Compare:

• Tra la teleskopo ni vidas krateron.
  = Through the telescope, we see a crater.
• Ni vidas krateron tra la teleskopo.
  = We see a crater through the telescope.

The first version may sound a bit more like the speaker wants to highlight how or by what viewpoint the crater is seen.

No—vidas by itself does not mean specifically we see.

In Esperanto, verbs do not change form for person:

• mi vidas = I see
• vi vidas = you see
• li vidas = he sees
• ni vidas = we see
• ili vidas = they see

So the pronoun is important unless the subject is already obvious from context.

That is why ni is included here.

Esperanto usually uses the simple present tense -as for both English see and are seeing, depending on context.

So:

• ni vidas can mean we see
• or we are seeing

Esperanto does not normally require a separate progressive form like English does. The simple present is enough in most cases.

Because it does not have the plural ending -j.

In Esperanto:

• kratero = crater
• krateroj = craters
• krateron = a crater (singular direct object)
• kraterojn = craters (plural direct object)

So krateron is clearly singular.

Yes. Because it has the accusative ending -n, its role stays clear even if you move it.

For example:

• Krateron ni vidas tra la teleskopo.
• Tra la teleskopo krateron ni vidas.

These are grammatically possible, but the most neutral order is usually closer to:

• Ni vidas krateron tra la teleskopo.

or, as in your sentence:

• Tra la teleskopo ni vidas krateron.

Changing the order usually changes focus or emphasis, not the core meaning.

Esperanto pronunciation is very regular.

teleskopo is pronounced roughly like:

teh-leh-SKO-po

A few useful points:

• e is always like e in met
• o is always like a clear o
• stress in Esperanto normally falls on the second-to-last syllable

So:

te - le - sko - po

The stress is on sko.

In this sentence it is most naturally literal: we are looking through a telescope.

More generally, tra can also be used in broader senses such as through, across, or by way of, but here the physical meaning is the obvious one.

A native English speaker might wonder whether it could mean something like by means of astronomy or from the telescope’s point of view. In normal usage, this sentence simply means that the telescope is the thing being looked through.