Learn Hardcore Swedish: Hon har mer energi på morgonen än på kvällen. - She has more energy in the morning than in the evening.

But that’s a slightly different wording. With har mer energi, you literally talk about “having more energy,” just like in English, so har is the natural choice here.

Why is it mer energi and not something like a comparative ending, or mera energi?

Mer is the regular comparative form of mycket (much) and många (many). You use mer in front of:

uncountable nouns: mer energi, mer vatten
adjectives with long forms: mer intressant, mer bekväm

There is no ending added to energi itself.

About mer vs mera:

mer energi – most common in modern Swedish, standard and neutral
mera energi – also correct, often a bit more informal or dialectal; you’ll hear it in speech and see it sometimes in writing

They mean the same thing in this context. You can safely use mer as your default.

Why do we say på morgonen and not i morgonen, when English has in the morning?

Swedish uses different prepositions for time than English, and you can’t translate them mechanically.

For parts of the day, the normal pattern is:

på morgonen – in the morning
på dagen – in the daytime
på eftermiddagen – in the afternoon
på kvällen – in the evening
på natten – at night

So here, på is simply the standard preposition used with these day‑parts in this meaning.

I is used with many other kinds of time expressions:

i januari – in January
i sommar – in the summer (this coming summer)
i år – this year

So på morgonen is just the idiomatic, correct combination for “in the morning.”

Why are morgonen and kvällen in the definite form (with -en)?

Morgon and kväll are both en‑words:

en morgon → morgonen
en kväll → kvällen

In time expressions, Swedish very often uses the definite form to talk about “the morning / the evening” in a general or typical sense:

på morgonen – in the morning (as a rule / usually)
på kvällen – in the evening

This doesn’t usually refer to one single, specific morning; instead it means “in the morning as a time of day” or “in the mornings (generally).”

If you want to refer to one particular morning or evening, you’d normally add another word or use the indefinite:

En morgon vaknade hon sent. – One morning she woke up late.
Den kvällen var det kallt. – That evening it was cold.

In habitual or general statements like your sentence, the definite form is the normal choice.

What exactly does än mean here, and how is it used with comparisons?

Än is the regular word used after a comparative to mean than:

större än – bigger than
snabbare än – faster than
mer energi än – more energy than

In your sentence:

mer energi … än … = more energy … than …

Important contrast:

än is used after comparisons: större än, mer än, bättre än
som is used after equality: lika stor som, samma som

So you say:

Hon har mer energi på morgonen än på kvällen.
She has more energy in the morning than in the evening.

but:

Hon har lika mycket energi på morgonen som på kvällen.
She has as much energy in the morning as in the evening.

Is the word order fixed, or can I say På morgonen har hon mer energi än på kvällen?

Both word orders are correct:

Hon har mer energi på morgonen än på kvällen.
På morgonen har hon mer energi än på kvällen.

Swedish main clauses follow the V2 rule: the finite verb (here har) must come in second position in the sentence.

In version 1, Hon is first, har is second.
In version 2, På morgonen is first, har is still second, and hon comes after.

So you can move på morgonen to the front for emphasis (putting more focus on the morning), as long as har stays in the second slot.

Could I also say Hon är mer energisk på morgonen än på kvällen? Is that the same?

Yes, that sentence is grammatically correct and natural:

Hon är mer energisk på morgonen än på kvällen.
= She is more energetic in the morning than in the evening.

Nuance:

har mer energi – literally “has more energy”; can feel a bit more physical/neutral.
är mer energisk – describes her as a more energetic person/behaviour at that time.

In many everyday contexts, they overlap and both would be fine. Your original sentence with har mer energi is slightly more neutral and very close to English.

How do you pronounce Hon har mer energi på morgonen än på kvällen?

Very approximate guidance (not strict IPA, just “sounds like”):

Hon – like hoon with a short oo
har – like English har in hard, but without the final d
mer – like English mare
energi – eh-nehr-GEE (stress on the last syllable)
på – like po with a long o (similar to English paw for many accents)
morgonen – roughly MORR-gon-en; the g can be quite soft or almost disappeared in casual speech
än – like en
kvällen – roughly KVEL-len; kv together, a bit like kv in kvetch

Said at normal speed, the sentence flows something like:

Hon har mer energi på morgonen än på kvällen.
[HON har meer ener-GEE po MORR-gon-en en po KVEL-len]

Why is it kvällen and not something like natten or aftonen?

The choice of word reflects which part of the day you mean:

kväll – evening (after late afternoon, before night)
natt – night (when people normally sleep)
afton – “evening” too, but more formal/old-fashioned or mostly used in fixed expressions like:
- god afton – good evening
- julafton – Christmas Eve
- midsommarafton – Midsummer’s Eve

Your sentence contrasts morning with evening, not with the deep night, so kvällen is the natural everyday word to use.

What’s the difference between på morgonen and på morgnarna? And similarly på kvällen / på kvällarna?

Both forms are possible, but there’s a nuance:

på morgonen – literally “on the morning” (singular, definite)
Often used to talk about mornings in general or “at that time of day” as a habit:
- Hon har mer energi på morgonen.
  She has more energy in the morning (as a rule).
på morgnarna – literally “on the mornings” (plural, definite)
Emphasises repeated instances, more like “in the mornings”:
- Hon tränar på morgnarna.
  She works out in the mornings (on those mornings, regularly).

In practice, both can express a general habit, and the difference is often subtle. The same pattern applies to evenings:

på kvällen – in the evening (as a time of day, generally)
på kvällarna – in the evenings (on those evenings, regularly)

Your sentence uses the very common, neutral singular definite forms på morgonen and på kvällen.

Is energi countable in Swedish? Could I say something like fler energier?

In this meaning (“physical/mental energy, stamina”), energi is normally treated as an uncountable noun in Swedish, just like energy usually is in English.

So you say:

mer energi – more energy
mycket energi – a lot of energy
lite energi – little energy

The plural energier exists but is rare and used in more specialized or abstract contexts (e.g. different “energies” in physics, esotericism, etc.). In everyday speech about how tired or alert someone is, you do not say fler energier; you use mer energi.