Plerique post longum iter statim dormiunt, sed una puella vigilat et per fenestram glaciem spectat.

Plerique means most or most people here. In Latin, it can be used substantively, meaning the adjective stands on its own without a noun. So plerique dormiunt means most people sleep or most of them sleep.

It is also a masculine plural nominative form. Latin often uses the masculine plural for a mixed or unspecified group of people.

Because it follows post, and post takes the accusative case. So longum iter means after a long journey.

A couple of details:

• iter is a neuter noun.
• Neuter nouns have the same form in the nominative and accusative singular, so iter stays iter.
• longum agrees with iter, so it is also accusative singular neuter.

Statim means immediately, at once, or right away. It goes with dormiunt, so the idea is they sleep immediately or they go to sleep right away.

Latin word order is flexible, so an adverb like statim can often move around without changing the basic meaning.

Una means one and agrees with puella in gender, number, and case. So una puella literally means one girl.

In context, it often has a slightly contrastive force: most people do one thing, but one girl does something different. So one girl or a single girl is a good sense here.

No. Classical Latin does not have articles like English the or a/an. That means:

• puella can mean girl, a girl, or the girl
• fenestram can mean window, a window, or the window

The context tells you which English article sounds best. Here, English usually says one girl and the window.

Vigilat means is awake, stays awake, or sometimes keeps watch. It does not mean looks at. That idea comes later with spectat.

So the sentence first says that the girl is awake, and then says what she is doing while awake.

Because the subject is still una puella. Latin does not need to repeat the subject if it is clear.

So:

• una puella vigilat = one girl is awake
• et per fenestram glaciem spectat = and she looks at the ice through the window

Latin can leave she unstated because the verb ending and context make the subject clear.

Because it follows per, and per takes the accusative case. So per fenestram means through the window.

This is a normal prepositional phrase:

• per = through
• fenestram = accusative singular of fenestra

Because glaciem is the accusative singular, while glacies is the nominative singular. Here the word is the direct object of spectat, so it must be in the accusative.

So:

• glacies = ice as a subject
• glaciem = ice as an object

Spectat means looks at, watches, or observes. In this sentence, looks at is probably the most natural translation.

It is a third person singular present active indicative form, so it means she looks at or she is looking at.

They do two different jobs:

• sed means but and contrasts the two main parts of the sentence
• et means and and links the two actions of the girl

So the structure is:

• Most people ... sleep, but one girl ...
• the girl stays awake and looks at the ice

Latin word order is much more flexible than English word order because the endings show how the words function.

Here the sentence is arranged quite naturally:

• Plerique = subject
• post longum iter = prepositional phrase
• statim = adverb
• dormiunt = verb

Then:

• sed = contrast
• una puella = new subject
• vigilat = first verb
• et per fenestram glaciem spectat = second action

Even if some parts were rearranged, the cases and verb endings would still show the relationships. The chosen order simply sounds smooth and emphasizes the contrast between most people and one girl.