Learn Hardcore Icelandic: Henni líður betur eftir klukkutíma.

Because the verb líða (to feel, in the sense of how one is doing) takes the experiencer in the dative case. So you say Henni líður…, Mér líður…, Þér líður…, etc.

Forms of “she”: nominative hún, accusative hana, dative henni, genitive hennar.
Note: Hún líður would not mean “she feels”; it suggests “she suffers/is dying” in a different sense of the verb and is not how you talk about feelings.

What exactly does líður mean here, and how is it used?

Líða has two common meanings:

Impersonal “feel” with a dative experiencer: Mér líður vel/illa = I feel well/bad.
“Pass (time)”: Tíminn líður hratt = Time passes quickly. In your sentence, it’s the first meaning: Henni líður… = She feels…

Why is it betur and not betri?

Betur is an adverb (comparative of vel “well”), and it modifies the verb líður. Use adverbs with how-you-feel expressions:

Henni líður vel/illa/betur/best. By contrast, betri is an adjective (comparative of góður) and modifies nouns: hún er betri = she is better (as a state/condition), not about how she feels.

Does eftir klukkutíma mean “in an hour” or “after an hour”?

Both are possible translations; Icelandic eftir + time (acc.) often corresponds to English “in” for future reference. Context decides:

As a future: “She will feel better in an hour.”
As a sequence after some event: “She feels better after an hour (has passed).” Present tense in Icelandic commonly refers to the near future.

What case is klukkutíma in, and why?

Accusative singular. With time expressions, eftir governs the accusative. The base noun is klukkutími (a clock-hour):

sg: nom klukkutími, acc/dat/gen klukkutíma
pl: nom klukkutímar, acc klukkutíma, dat klukkutímum, gen klukkutíma

Is an article like “an” required in Icelandic (“an hour”)?

No. Icelandic has no indefinite article. Eftir klukkutíma already means “after/in an hour.” You can add the numeral for emphasis: eftir einn klukkutíma (still accusative), but it isn’t necessary.

How do I say it in the past or make a clear future?

Past: Henni leið betur eftir klukkutíma. (She felt better after an hour.)
Clear future (with munu): Henni mun líða betur eftir klukkutíma. (She will feel better in an hour.)
Everyday speech often just uses present for near future: Henni líður betur eftir klukkutíma.

Can I use other words for “hour,” like klukkustund?

Yes. Klukkustund is a common synonym.

Eftir klukkustund (accusative fem. sg.) = in/after an hour.
With the numeral: eftir eina klukkustund. Both klukkutími and klukkustund are natural here.

What’s the difference between eftir klukkutíma and í klukkutíma?

Eftir klukkutíma = in an hour (from now) / after an hour has passed.
Í klukkutíma = for an hour (duration). Compare:
Henni líður betur eftir klukkutíma. (She will feel better in an hour.)
Henni leið illa í klukkutíma. (She felt bad for an hour.)

What about á klukkutíma and innan klukkutíma?

Á klukkutíma = in the span of an hour (used for how long something takes to complete): Hún náði bata á klukkutíma (she recovered within/over the course of an hour).
Innan klukkutíma = within an hour (before an hour elapses): Henni mun batna innan klukkutíma.

How do I ask or negate this sentence?

Yes/no question (verb-second): Líður henni betur eftir klukkutíma?
Wh-question: Hvenær líður henni betur?
Negation: place ekki after the finite verb: Henni líður ekki betur eftir klukkutíma.

What are some common patterns with líða for feelings?

Mér líður vel/illa. (I feel good/bad.)
Hvernig líður þér? (How do you feel?)
Henni hefur liðið illa. (She has felt bad.) These all use a dative experiencer (mér/þér/honni/henni…).

Can I change the word order?

Yes, Icelandic is verb-second. Whatever you front, the finite verb (líður) stays in second position:

Henni líður betur eftir klukkutíma. (subject-like phrase first)
Eftir klukkutíma líður henni betur. (time first) Both are fine and natural.

How do I say “later” without a number?

Use síðar or seinna:

Henni líður betur síðar/seinna. = She feels better later. With a time phrase, you can also say: klukkutíma síðar = an hour later.

Any quick pronunciation tips for the sentence?

Roughly:

Henni [ˈhɛnːɪ] (double n = long n)
líður [ˈliːðʏr] (í is long [iː]; ð is a soft “th” as in “this”)
betur [ˈpɛːtʏr] (stress on first syllable)
klukkutíma [ˈkʰlʏhʏtʰiːma] (kk is pre-aspirated, sounding like [hk]; stress on the first syllable of each word) Primary stress is always on the first syllable of Icelandic words.

Does eftir always take the accusative?

Not always. With time/direction meanings like “after/in (time),” it takes the accusative: eftir klukkutíma. But with meanings like “according to/depending on,” it takes the dative:

Það fer eftir því. (It depends on it.) — dative því.