Illalla olen pitkään kylvyssä rentoutuakseni.

Illalla means in the evening or during the evening.

It comes from ilta = evening.
The ending -lla is the adessive case, and one of its common uses is with times:

• aamulla = in the morning
• päivällä = in the daytime
• illalla = in the evening
• yöllä = at night

So this is not literally on the evening in the English sense; it is just the normal Finnish way to express time here.

Because Finnish usually does not need the subject pronoun when the verb already shows who the subject is.

• olen = I am

So olen already tells you the subject is I, and adding minä would usually only be for emphasis, contrast, or clarity.

Compare:

• Olen kylvyssä. = I am in the bath.
• Minä olen kylvyssä. = I am in the bath (with extra emphasis on I)

Pitkään means for a long time.

Here it works as an adverb of duration. It tells you how long the speaker is in the bath.

• Olen pitkään kylvyssä. = I am in the bath for a long time.

It is related to the adjective pitkä = long, but in this form it functions adverbially, not as an adjective describing a noun.

Because kylvyssä means in the bath, describing a location or state, while kylpyyn would mean into the bath, describing movement.

• kylvyssä = in the bath
• kylpyyn = into the bath

So:

• Olen kylvyssä. = I am in the bath.
• Menen kylpyyn. = I go into the bath.

In your sentence, the speaker is already there, so kylvyssä is the correct form.

This is because Finnish word stems often change when endings are added.

The dictionary form is:

• kylpy = bath

But in many case forms, the stem changes to kylvy-, giving:

• kylvyssä = in the bath
• kylvystä = from the bath

This is a normal type of Finnish stem alternation. A learner usually just needs to get used to the fact that the stem of kylpy is often kylvy- before case endings.

Rentoutuakseni means to relax, so that I can relax, or in order to relax.

It expresses purpose. In this sentence, it tells you why the speaker is in the bath for a long time.

So the structure is roughly:

• Illalla olen pitkään kylvyssä rentoutuakseni.
• In the evening, I stay in the bath for a long time in order to relax.

This form is more purpose-focused than a plain English to relax might suggest.

It is built from the verb rentoutua = to relax.

The form breaks down like this:

• rentoutua = to relax
• rentoutuakse- = in order to relax
• rentoutuakseni = in order for me to relax

The ending -kse- is the key part that gives the meaning in order to.
Then -ni shows that the subject of that purpose is I.

So literally, it is something like:

• for me to relax
• in order that I relax

The -ni is a possessive suffix, and here it shows who is meant to do the relaxing.

• -ni = my / I
• in this structure, it means the purpose belongs to me

So:

• rentoutuakseni = in order for me to relax

This is very normal in Finnish with this kind of purpose expression. English usually does not mark this separately, but Finnish does.

Not completely. Finnish word order is fairly flexible, but different orders change what sounds most natural or what gets emphasis.

Your sentence starts with Illalla, which sets the time frame first:

• Illalla olen pitkään kylvyssä rentoutuakseni.

That sounds natural, like As for the evening / In the evening, I...

You could also hear other orders, for example:

• Olen illalla pitkään kylvyssä rentoutuakseni.

That is also understandable, but it puts the focus a little differently. Finnish often places the most topical or important information early in the sentence.

Not very well. A very literal version would sound stiff in English:

• In the evening I am for a long time in the bath in order to relax.

Natural English would usually say something more like:

• In the evening, I spend a long time in the bath to relax.
• In the evening, I stay in the bath for a long time to relax.
• In the evening, I take a long bath to relax.

So the Finnish sentence is completely normal, but a direct word-for-word English version is not the best way to understand its style.

Yes, but the meaning and structure would change.

For example:

• Menen kylpyyn rentoutumaan. = I go into the bath to relax.

Here rentoutumaan is used with a verb of movement, because the idea is going somewhere to do something.

But in your original sentence:

• olen kylvyssä rentoutuakseni

the main verb is olen = I am, so the sentence is about being in the bath, not going into it. That is why rentoutuakseni fits well: it expresses purpose without focusing on movement.

So both are correct, but they are used in different kinds of sentences.