Learn Hardcore Finnish: Vihkon lopussa on pieni lista sanoja ja lyhyt selitys siitä, mitä ne tarkoittavat. - At the end of the notebook there is a small list of words and a short explanation of what they mean.

So the structure is [thing] + genitive + [part that has a location] + inessive:
vihkon lopussa = “in the end of the notebook”, “at the end of the notebook”.

You can’t say vihkon loppu here, because you need the idea of location (“in/at the end”).
Vihkossa would mean “in the notebook” in general, not specifically “at the end”.

What case is lopussa, and what’s the difference between lopussa and loppuun?

Lopussa is in the inessive case (-ssa/-ssä) and means “in/at the end”.

loppu = end
lopussa = “in/at the end” (static location)
loppuun = illative (“into/to the end”) – movement towards the end

In this sentence we describe where something is, not where it moves, so lopussa is correct, not loppuun.

Why is it pieni lista sanoja and not pieni lista sanat or pieni lista sanoista?

Sanoja is partitive plural of sana (word).

pieni lista sanoja = “a small list of words” (an indefinite collection)
sanat (nominative plural) would sound like you are naming the items: “a small list: the words …”
sanoista (elative plural “from/of the words”) is also possible: lista sanoista = “a list of the words (from a known set)”

Here pieni lista sanoja suggests just “some words”, not necessarily a specific fixed set, which matches a typical vocabulary list.

Can you explain the word order Vihkon lopussa on pieni lista… instead of Pieni lista on vihkon lopussa?

This is a typical existential sentence in Finnish:

[location] + on + [new / important thing]

So:

Vihkon lopussa on pieni lista… ≈ “In the end of the notebook there is a small list…”

The location vihkon lopussa comes first, because the speaker is presenting what exists there.
Pieni lista on vihkon lopussa is also grammatically correct, but it emphasizes “the small list” (as if we already know about the list and now say where it is).

What is the role of siitä in lyhyt selitys siitä, mitä ne tarkoittavat?

Siitä is the elative of se (“it / that”) and here it means roughly “about that” or “of that”.

lyhyt selitys = a short explanation
siitä, mitä ne tarkoittavat = “about what they mean”

Literally the structure is “a short explanation of that, what they mean”.
In natural English we just say “a short explanation of what they mean”, but Finnish often keeps that extra pronoun siitä.

Could you leave out siitä and just say lyhyt selitys, mitä ne tarkoittavat?

You might see lyhyt selitys, mitä ne tarkoittavat in informal speech, but:

In standard written Finnish, siitä is preferred and sounds more complete:
lyhyt selitys siitä, mitä ne tarkoittavat

Without siitä, the clause mitä ne tarkoittavat feels more like a direct question embedded in the sentence (“a short explanation: what do they mean?”), which is less natural in formal style.

Why is there a comma before mitä?

Because mitä ne tarkoittavat is a subordinate clause (a dependent clause) that explains siitä (“of that”).

Finnish usually separates a main clause and a following subordinate clause with a comma:

lyhyt selitys siitä, mitä ne tarkoittavat
= “a short explanation of what they mean”

So the comma works similarly to English:
“a short explanation of what they mean” → in Finnish: siitä, mitä ne tarkoittavat with a comma.

Why is it mitä here and not jota or joka?

Mitä is used here as a relative/interrogative pronoun meaning “what” in clauses like “what they mean”.

mitä is typically used for “what” (not referring to a specific noun mentioned before)
joka/jota are “which/that” referring to a specific noun (the one mentioned before)

Compare:

selitys siitä, mitä ne tarkoittavat
= “an explanation of what they mean” (general “what”)
selitys sanoista, jotka ovat vaikeita
= “an explanation of the words that are difficult” (jotka refers to sanoista)

So mitä is the natural choice after “explanation of what…”.

Why is it mitä ne tarkoittavat and not mitä se tarkoittaa?

Because the subject of tarkoittavat is ne (“they”), referring back to the words (sanoja).

ne = “they” (plural)
tarkoittavat = 3rd person plural form of tarkoittaa

If we used se tarkoittaa (“it means”), we would be talking about a single thing.
Here we have multiple words, so Finnish uses the plural subject ne and plural verb tarkoittavat: “what they mean”.

Why is the verb tarkoittavat in the plural?

In Finnish, the verb agrees with its subject in person and number.

Subject: ne (“they” – 3rd person plural)
Verb: tarkoittavat (3rd person plural form of tarkoittaa)

So ne tarkoittavat = “they mean”.
Using a singular verb (ne tarkoittaa) would be casual/colloquial in some dialects, but in standard Finnish ne tarkoittavat is correct.

Could we say selitys siitä, mitä sanat tarkoittavat instead of mitä ne tarkoittavat?

Yes, that is also correct:

mitä ne tarkoittavat = “what they mean” (using the pronoun ne)
mitä sanat tarkoittavat = “what the words mean”

The meaning is essentially the same.
Using ne is slightly lighter and more natural here, because sanoja has just been mentioned and ne smoothly refers back to those words.

What is the difference between vihko and kirja for “book/notebook”?

vihko = a notebook / exercise book, often with blank or lined pages for writing, notes, homework, etc.
kirja = a book in the more general sense (novel, textbook, etc.)

So vihko is the right word here if we’re talking about a school exercise book or a notebook that has a word list at the end.

Could the sentence be reordered as Pieni lista sanoja ja lyhyt selitys siitä, mitä ne tarkoittavat, on vihkon lopussa?

Yes, that is grammatically correct:

Pieni lista sanoja ja lyhyt selitys siitä, mitä ne tarkoittavat, on vihkon lopussa.

This version emphasizes the list and the explanation first, then tells you where they are.
The original word order Vihkon lopussa on… emphasizes the location first and then introduces what is found there. Both are fine; the choice is about emphasis and style.