Learn Hardcore Finnish: Pitkän päivän jälkeen väsymys tuntuu selässäni. - After a long day, I can feel the tiredness in my back.

X + genitive + jälkeen = after X
- pitkä päivä → pitkän päivän jälkeen = after a long day
- kurssi → kurssin jälkeen = after the course

Because the adjective must agree in case with the noun, both pitkä and päivä get the genitive ending: pitkän päivän jälkeen.

What exactly does jälkeen do, and where does it go in the sentence?

Jälkeen is a postposition meaning after. Unlike English prepositions, it comes after the word it governs, not before it.

Structure:

[Genitive phrase] + jälkeen
- pitkän päivän jälkeen = after a long day
- tunnin jälkeen = after an hour

In your sentence, pitkän päivän jälkeen is a time expression at the beginning: After a long day, …

Why is väsymys used (a noun: tiredness) instead of the adjective väsynyt (tired)?

Finnish often uses abstract nouns + verb to talk about sensations:

väsymys tuntuu = tiredness is felt / you can feel the tiredness
kipu tuntuu = the pain is felt

Using väsymys focuses on the feeling as a thing located in your body.
If you say olen väsynyt, you are just stating your overall state: I am tired.
Väsymys tuntuu selässäni adds where you especially feel that tiredness: in my back.

Why is the verb tuntuu used instead of something like tunnen?

Finnish has two different verbs:

tuntua = to be felt, to feel (like)
tuntea = to feel (something), to know, to be acquainted with

In your sentence, väsymys is the subject, and we describe how it is experienced:

väsymys tuntuu selässäni = the tiredness is felt in my back

If you used tuntea, it would need an object and would sound like you are deliberately feeling tiredness, which is not how sensations are normally expressed:

tunnen väsymyksen selässäni = grammatically possible but heavy and uncommon in normal speech.

Who is the subject: is it väsymys or “I”?

The grammatical subject is väsymys (nominative singular). The verb tuntuu is 3rd person singular to agree with it.

The experiencer (whose back? who feels it?) is expressed indirectly by the possessive suffix -ni in selässäni (in my back).

So structurally the sentence is:

(After a long day) tiredness is felt in my back.

Finnish often uses this kind of structure where the sensation itself is the subject, not I.

What exactly does selässäni mean, and how is it formed?

Selässäni breaks down like this:

selkä = back
-ssä / -ssä (inessive case) = in, inside, in the area of → selässä = in (the) back
-ni (1st person singular possessive suffix) = my → selässäni = in my back

So selässäni literally means in my back and tells both location (inessive) and owner (possessive suffix).

Why is it selässäni (inessive) and not something like selkääni?

Different cases give slightly different ways of talking about body sensations:

selässäni (inessive) = in my back, as a location
- väsymys tuntuu selässäni = I feel the tiredness in my back.
selkääni (illative, often partitive + possessive) is used with many pain verbs:
- selkääni särkee / kolottaa = my back aches / is sore.

With tuntua, the inessive (selässäni) is natural, because you’re describing where the feeling is present, not something entering there.

Could I leave out the possessive suffix and just say selässä? What changes?

Yes, grammatically you could say:

Pitkän päivän jälkeen väsymys tuntuu selässä.

This would mean tiredness is felt in the back, but it sounds more impersonal or generic, not clearly my back.

With selässäni, it’s clearly in my back.
Normally, for your own body parts, the possessive suffix (selässäni, päässäni, jaloissani) is the most natural choice.

Can I use minun instead of the possessive suffix, like minun selässä?

You should not say minun selässä without the suffix; that is felt incomplete. The fully marked form is:

minun selässäni = in my back

In everyday Finnish, you usually use either:

just the suffix: selässäni (most common), or
pronoun + suffix for emphasis/contrast: minun selässäni (e.g. “in my back, not someone else’s”).

So:

Natural: väsymys tuntuu selässäni
Also possible (more emphatic): väsymys tuntuu minun selässäni
Unnatural: väsymys tuntuu minun selässä

Can the word order be changed, and does it change the meaning?

Yes, Finnish word order is relatively flexible. All of these are possible:

Pitkän päivän jälkeen väsymys tuntuu selässäni.
Pitkän päivän jälkeen selässäni tuntuu väsymys.
Väsymys tuntuu selässäni pitkän päivän jälkeen.

The basic meaning stays the same, but the emphasis shifts:

First version: neutral, time → subject → verb → place.
Second: puts more focus on your back (selässäni) as the place where it is felt.
Third: foregrounds väsymys tuntuu selässäni, and the time phrase feels slightly more “added on” at the end.

Could I say something like Pitkän päivän jälkeen olen väsynyt selässäni?

That sounds very odd in Finnish. Olla väsynyt describes a person’s overall state, not a specific body part. Saying “I am tired in my back” doesn’t really fit Finnish patterns.

More natural alternatives:

Pitkän päivän jälkeen selkäni on väsynyt. = After a long day, my back is tired.
Pitkän päivän jälkeen väsymys tuntuu selässäni. = After a long day, I feel the tiredness in my back.
For pain: Pitkän päivän jälkeen selkääni särkee. = After a long day, my back aches.

How would I negate this sentence?

You negate it with the negative verb ei and the connegative form of tuntua (tunnu):

Pitkän päivän jälkeen väsymys ei tunnu selässäni.
= After a long day, the tiredness is not felt in my back.

Structure:

ei (3rd person singular negative) + tunnu (connegative form of tuntua)
Everything else stays the same.

You can intensify it by adding e.g. ollenkaan:

väsymys ei tunnu selässäni ollenkaan = I don’t feel the tiredness in my back at all.