Learn Hardcore Esperanto: En vintro, kiam neĝo kuŝas sur la tero, nia hejmo ŝajnas plej varma. - In winter, when snow lies on the ground, our home seems warmest.

en vintro = in (the) winter, during wintertime in general
en la vintro would sound more like in that specific winter (e.g. a particular year’s winter you already have in mind).

So for a general statement about what usually happens in winter, en vintro is the normal choice, even though English uses the.

Could we say vintre instead of en vintro? What is the difference?

Yes, you can say:

Vintre, kiam neĝo kuŝas sur la tero, nia hejmo ŝajnas plej varma.

vintre is an adverb meaning in winter / during wintertime. It is a bit more compact and slightly more stylistic or literary than en vintro, but the meaning here is practically the same.

So:

en vintro = in winter (literally, in winter as a noun phrase)
vintre = in wintertime (the season used as an adverb)

Both are correct and idiomatic.

Can I also say dum vintro? How does that compare to en vintro?

Yes:

Dum vintro literally emphasizes the duration: during winter.
En vintro is a bit more neutral: in winter.

In most contexts (including this sentence), en vintro and dum vintro can both be used without a real change in meaning. dum just puts slightly more focus on the time span as a period.

Why does neĝo have no article? Why not la neĝo?

In Esperanto, mass nouns and uncountable substances often appear without la when you are talking about them in a general, indefinite way.

neĝo = snow in general (some snow, snow as a phenomenon)
la neĝo = the snow (specific snow already known or just mentioned)

Here, the sentence describes a typical winter situation in general, so neĝo without la is natural: when snow lies on the ground.

You could say la neĝo if you have some particular snow in mind (for example, the snow we see outside right now), but the general-proverbial tone of the sentence favors plain neĝo.

What exactly does kuŝas mean here? Why not simply estas?

kuŝi means to lie, to be lying, to rest (in a lying position).

neĝo kuŝas sur la tero = snow lies on the ground (spreads out, is resting on the surface)
neĝo estas sur la tero = snow is on the ground (more neutral, just states its presence)

Both are grammatically correct, but kuŝas creates a more visual, descriptive image of snow lying over the earth. It is a style choice that makes the sentence more vivid.

Why is it sur la tero with la, if neĝo and vintro do not use la?

Here la tero refers to the ground / the earth’s surface, which is treated more like a specific thing than like a general, abstract noun.

Compare:

neĝo = snow as a substance (general phenomenon)
vintro = winter as a season in general
la tero = the ground / the Earth (a particular, definite thing we are imagining the snow lying on)

You could sometimes see sur tero used in a more abstract way (on earth / on any ground), but sur la tero is more natural when you picture a specific scene with snow lying on the ground.

Why is there no accusative -n anywhere? Is that because every noun here is a subject or after a preposition?

Exactly.

In the sentence:

En vintro – prepositional phrase (no -n)
kiam neĝo kuŝas sur la tero – neĝo is the subject; sur la tero is a prepositional phrase
nia hejmo – subject of ŝajnas
ŝajnas plej varma – varma is a predicate adjective, not a direct object

The accusative -n is used mainly for direct objects or for some time/space expressions. Here, there is no direct object and all nouns (except subjects) are governed by prepositions (en, sur), so no -n appears.

What does hejmo add compared to domo? Could I say nia domo instead of nia hejmo?

You could say nia domo, but there is a nuance:

domo = house, a building
hejmo = home, the place where you live and feel at home (more emotional)

So:

Nia hejmo ŝajnas plej varma = Our home seems warmest (emotional, cozy feeling)
Nia domo ŝajnas plej varma = Our house seems warmest (talking a bit more about the building itself)

The original sentence emphasizes the feeling of home, not just the structure.

How does ŝajnas work with adjectives? Why is there no esti (like ŝajnas esti varma)?

In Esperanto, ŝajni works similarly to English to seem / to appear and can directly take a predicate adjective, without esti:

Nia hejmo ŝajnas varma. = Our home seems warm.
Nia hejmo ŝajnas tre komforta. = Our home seems very comfortable.

You can say ŝajnas esti varma, but that is usually more formal or heavier in style. In everyday language, ŝajnas varma is simpler and fully correct.

So the pattern is:

io ŝajnas [adjektivo] = something seems [adjective]

Why is it plej varma and not la plej varma? I thought the superlative needed la.

plej forms the superlative (most, -est), and la plej varma is certainly correct: la marks this as the warmest one among others.

However, in predicate position (after estas, ŝajnas, etc.), Esperanto often omits la when the context already clearly defines what we are talking about:

Nia hejmo estas plej varma.
Nia hejmo ŝajnas plej varma.

These mean essentially the same as …estas la plej varma / ŝajnas la plej varma in many contexts: Our home is (the) warmest / seems (the) warmest.

So:

la plej varma – explicitly the warmest
plej varma – warmest / very warm in comparison, with la often omitted in predicate use.

Both are correct here; plej varma sounds natural and slightly less heavy.

Why is the present tense ŝajnas used? Shouldn’t it be something like ŝajnas only if it’s happening right now?

In Esperanto, just like in English, the simple present can express:

What is happening now
- Mi legas. – I am reading (right now).
General truths or habitual situations
- Printempe pluvas ofte. – In spring it often rains.

Our sentence is of type 2: a general, habitual situation in winter. So present tense ŝajnas fits perfectly:

En vintro … nia hejmo ŝajnas plej varma.
= In winter, … our home seems warmest (as a general fact).

You would not normally use another tense here.

Can I change the word order, for example: Nia hejmo ŝajnas plej varma en vintro, kiam neĝo kuŝas sur la tero?

Yes, Esperanto word order is relatively flexible, especially for time and place phrases.

All of these are grammatically fine and mean the same thing:

En vintro, kiam neĝo kuŝas sur la tero, nia hejmo ŝajnas plej varma.
Nia hejmo ŝajnas plej varma en vintro, kiam neĝo kuŝas sur la tero.
Nia hejmo, en vintro, kiam neĝo kuŝas sur la tero, ŝajnas plej varma. (more marked, but still possible)

What changes is emphasis and style, not basic meaning. Placing En vintro first highlights the time frame as the topic.

Are the commas in the sentence important? How do they relate to the clause kiam neĝo kuŝas sur la tero?

The commas help to separate the parts:

En vintro, – introductory time phrase
kiam neĝo kuŝas sur la tero, – subordinate clause (when snow lies on the ground)
nia hejmo ŝajnas plej varma. – main clause

You would almost always put a comma before and after a kiam-clause when it is inserted like this. This makes it clear where the subordinate clause begins and ends.

In speech, the commas correspond to small pauses. In writing, they just improve clarity; the underlying grammar (who is subject, which verb goes with which part) does not depend on the commas alone.