Learn Hardcore Swedish: Normalt tränar hon på kvällen, men idag tränar hon på morgonen. - Normally she works out in the evening, but today she works out in the morning.

If the subject comes first:
Hon tränar normalt på kvällen.
(Subject = place 1, verb = place 2)
If you move an adverbial (like Normalt) to the beginning, the verb still has to be in second place:
Normalt tränar hon på kvällen.
(Adverbial = place 1, verb = place 2, subject = place 3)

So Normalt hon tränar is wrong, because the verb is no longer in second position.

Could we also say Hon tränar normalt på kvällen? Is there a difference in meaning?

Yes, Hon tränar normalt på kvällen is perfectly correct.

The difference is mainly what you emphasize:

Normalt tränar hon på kvällen
– Slight emphasis on what is normal vs today. You're contrasting normalt with idag.
Hon tränar normalt på kvällen
– A more neutral, “plain” statement about her usual habit.

Both mean essentially the same thing; the first one just foregrounds the idea of “normally” a bit more.

Why do we use på in på kvällen and på morgonen instead of something like i?

For general times of day, Swedish typically uses på + definite form:

på morgonen = in the morning
på dagen = in the daytime
på kvällen = in the evening
på natten = at night

Using i here (i kvällen, i morgonen) is incorrect in standard Swedish for this meaning.

So the idiomatic way to say “in the evening / in the morning” (as a time of day) is på kvällen and på morgonen.

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

In Swedish, when you talk about parts of the day in a general, habitual way, you normally use the definite form:

på kvällen = in the evening (in general, as a time of day)
på morgonen = in the morning
på sommaren = in (the) summer

It does not mean “that specific evening” here. It’s just the normal way Swedish expresses a general time frame.

So:

Hon tränar på kvällen.
= She works out in the evenings / at night (as a habit).

Using kväll or morgon in the indefinite singular (på kväll, på morgon) is ungrammatical in this time-expression pattern.

Does tränar here mean “works out” or “is working out”? Why is the same form used?

The Swedish present tense (tränar) covers both English simple present and present progressive:

Normalt tränar hon på kvällen.
= She normally works out in the evening. (habitual)
… men idag tränar hon på morgonen.
= but today she is working out in the morning. (right now / today)

Swedish typically doesn’t use a special “-ing” form for ongoing actions in the present. Context words like normalt and idag tell you whether it’s habitual or happening now.

Could we use brukar instead of normalt? For example: Hon brukar träna på kvällen?

Yes, you can, and it’s very natural:

Hon brukar träna på kvällen.
≈ She usually / normally works out in the evening.

Difference in nuance:

Normalt tränar hon på kvällen
– Adverb normalt (“normally”) modifies the whole clause. It’s more neutral and descriptive.
Hon brukar träna på kvällen
– brukar
- infinitive (träna) directly expresses habit. It emphasizes that this is her customary behavior.

In many contexts they are interchangeable, but brukar is specifically about habits, while normalt can also refer more broadly to what is “normal” or “typical” in a situation.

Why is it men idag tränar hon and not men idag hon tränar?

This is again the V2 rule (verb in second position):

men idag tränar hon …
– idag is first, so the verb tränar must come second, then the subject hon.

men idag hon tränar is ungrammatical, because the verb is no longer in second place.

Note that you can also say:

men hon tränar idag på morgonen.

Here, hon is first, so tränar is still in second position, which is allowed. The difference is just emphasis / focus, not grammar.

Is the comma before men in …, men idag tränar hon … required in Swedish?

Putting a comma before men in this kind of sentence is normal and recommended in Swedish:

Normalt tränar hon på kvällen, men idag tränar hon på morgonen.

You generally:

Do put a comma before men when it starts a new main clause that contrasts with the previous one.
Do not put a comma before och as often as in English (Swedish comma rules are a bit stricter), but men usually gets one when it links two full clauses.

So in this sentence, the comma is standard and natural.

Why is idag written as one word? Can I also write i dag?

Both spellings exist:

idag (one word) – now the most common, modern spelling.
i dag (two words) – also correct, somewhat more traditional.

There is no difference in meaning. It’s just a spelling preference.

The same applies to some other time words:

imorgon / i morgon (tomorrow)
igår / i går (yesterday)

In contemporary, informal writing, idag / imorgon / igår as one word is extremely common.

What is the difference between på kvällen and på kvällarna when talking about habits?

Both can describe habits, but with a slight nuance:

Hon tränar på kvällen.
– Very common way to say she usually works out in the evening (as a general time of day). Neutral, habitual.
Hon tränar på kvällarna.
– Literally “on the evenings”. It can make you think a bit more of repeated, individual evenings (“in the evenings (plural)”), sometimes with a sense of most evenings.

In many contexts they can both translate as “in the evenings”, and both can describe a routine.
Your sentence with på kvällen is completely natural Swedish for talking about a regular habit.