La montpasejo estas parto de nia itinero.

It is a compound word, which is very common in Esperanto.

• mont- = mountain
• pas- = pass, go through
• -ej- = place
• -o = noun ending

So montpasejo literally means something like a place for passing through mountains, which is why it means mountain pass or mountain passage.

Esperanto often builds words this way, so learning to spot the parts is very helpful.

La is the definite article, meaning the.

So La montpasejo means the mountain pass, not just a mountain pass.

Esperanto has only one definite article: la. It does not change for gender, number, or case.

Estas is the present-tense form of esti, which means to be.

So:

• estas = is / are / am

In this sentence, it means is:

• La montpasejo estas parto... = The mountain pass is part...

Esperanto verbs do not change according to the subject, so estas stays the same with mi, vi, ĝi, ili, and so on.

Because parto is not a direct object here.

After esti (to be), the word that follows usually describes or identifies the subject, so it stays in the basic noun form:

• La montpasejo estas parto...

The -n ending is mainly used for direct objects and for motion toward something. Neither of those applies here.

So:

• parto = correct
• parton = not correct in this sentence

Here de means of.

So:

• parto de nia itinero = part of our route/itinerary

This is a very common Esperanto pattern:

• la fino de la libro = the end of the book
• parto de la urbo = part of the city
• amiko de Maria = a friend of Maria

Because itinero is singular, and in this sentence it is part of a de phrase, not a direct object.

• nia = our
• itinero = route / itinerary

Why not niaj?
Because niaj would have to agree with a plural noun. But itinero is singular.

Why not nian itineron?
Because the accusative -n is not needed after de. In Esperanto, words after prepositions normally do not take -n unless there is a special reason.

So de nia itinero is exactly what you want.

Yes. They are closely related in meaning and form.

• itinero = route, itinerary
• English itinerary has the same Latin background

This is one of many Esperanto words that can feel familiar to English speakers, especially if they know words from French- or Latin-based vocabulary.

Yes, Esperanto word order is fairly flexible, but some orders sound more natural than others.

The normal, clear order here is:

• La montpasejo estas parto de nia itinero.

You could also say:

• Parto de nia itinero estas la montpasejo.

That changes the emphasis a bit, putting parto de nia itinero first.

Because Esperanto uses endings to show grammar, word order is freer than in English, but the most neutral style is still usually subject + verb + complement.

Because the sentence is talking about one mountain pass.

• montpasejo = one mountain pass
• montpasejoj = mountain passes

If the sentence meant more than one, it would be:

• La montpasejoj estas parto de nia itinero.

So the lack of -j tells you it is singular.

In Esperanto, stress almost always falls on the second-to-last syllable.

So the main stress is:

• La montpaSEjo ES tas PARto de NIa itiNEro

More specifically:

• montpasejo → stress on SE
• parto → stress on PAR
• itinero → stress on NE

That regular stress system makes pronunciation much more predictable than in English.