Learn Hardcore Finnish: Hänen tyttöystävänsä sanoo usein, että poikaystävä on liian stressaantunut töistä. - His/Her girlfriend often says that the boyfriend is too stressed by work.

Hänen – the possessor pronoun in the genitive (hänen = his / her).
-nsä – the third‑person possessive suffix attached to the possessed noun:
tyttöystävä → tyttöystävänsä = his/her girlfriend.

So Hänen tyttöystävänsä literally means his/her girlfriend-of-his/hers.

Important points:

In spoken Finnish, people often drop the suffix and just say hänen tyttöystävä, but in formal/standard written Finnish the suffix -nsä is expected.
You can also drop Hänen and keep only the suffix:
Tyttöystävänsä sanoo usein… = His/her girlfriend often says…
The suffix -nsä alone still tells you someone owns the girlfriend, and context gives you who.

So in careful written Finnish, having both Hänen and -nsä is normal, not redundant.

Could we say just Tyttöystävänsä sanoo usein… without Hänen?

Yes.

Tyttöystävänsä sanoo usein, että…
also means His/her girlfriend often says that…

Here tyttöystävänsä already contains the possessive suffix -nsä, so it grammatically means someone’s girlfriend. If the context is clear (we already know who “he/she” is), you don’t need Hänen.

Nuance:

With Hänen tyttöystävänsä, the pronoun Hänen makes the reference a bit clearer and more explicit.
Without Hänen, it’s still grammatical; it can sound slightly more compact or literary.

Why is tyttöystävänsä in this form and not, for example, tyttöystävä?

The base word is tyttöystävä (girlfriend). In the sentence it appears as:

tyttöystävä (nominative singular, dictionary form)
tyttöystävänsä = tyttöystävä
- -n (genitive stem) + -nsä (possessive suffix)

Why this form?

It is the subject of the main clause, so structurally it could be nominative.
But standard Finnish typically attaches the possessive suffix to the genitive stem of the noun:
tyttöystävä → stem tyttöystävä- → tyttöystävän
- -sä → tyttöystävänsä.

So tyttöystävänsä functions as a possessed subject: his/her girlfriend.

Why is it sanoo usein and not usein sanoo? Can the word order change?

Both are possible:

Hänen tyttöystävänsä sanoo usein, että…
Hänen tyttöystävänsä usein sanoo, että…

Meaning-wise, both are His/her girlfriend often says that….

Nuance:

sanoo usein = neutral placement: verb first, then the adverb usein.
usein sanoo = puts a bit more emphasis on how often it happens (the “often” is more prominent).

Finnish word order is fairly flexible; adverbs like usein can usually move around the verb without changing the core meaning, only the emphasis.

What does että do here? Why do we need it?

Että is a subordinating conjunction meaning “that” (for reported speech, thoughts, etc.).

…sanoo usein, että poikaystävä on liian stressaantunut töistä.
= …often says that the boyfriend is too stressed from work.

Just like in English you need that in “She says that he is tired” (especially in careful speech), Finnish needs että to introduce the content clause (the thing being said). Without että, the sentence would be ungrammatical or at best sound broken.

Why is poikaystävä in the basic (nominative) form even though it’s in the second part of the sentence?

In the clause että poikaystävä on liian stressaantunut töistä, poikaystävä is the subject of that subordinate clause.

In Finnish, the subject normally appears in the nominative case (the dictionary form), so:

poikaystävä on… = the boyfriend is…

The fact that it’s inside an että‑clause doesn’t change its role as subject, so it stays in the nominative.

Shouldn’t it be hänen poikaystävänsä to match hänen tyttöystävänsä? Why is it just poikaystävä?

You can say:

Hänen tyttöystävänsä sanoo usein, että hänen poikaystävänsä on liian stressaantunut töistä.

This would clearly mean:
His/her girlfriend often says that her/his boyfriend is too stressed from work.

In the given sentence:

että poikaystävä on liian stressaantunut töistä

the word poikaystävä appears without a pronoun or possessive suffix. That makes it more like:

…that (the) boyfriend is too stressed from work (understood from context which boyfriend).

So:

With hänen poikaystävänsä = explicitly “his/her boyfriend”.
With bare poikaystävä = “the boyfriend” in this context; the possessor is only implied.

In natural, fully explicit standard Finnish, hänen poikaystävänsä would usually be preferred if you really want to tie him to that same “hänen”.

What does liian mean exactly, and how is it different from liikaa?

Both relate to the idea of “too much”, but they are used differently:

liian
- adjective/adverb
  → liian stressaantunut = too stressed
  → liian kallis = too expensive
  → liian nopeasti = too quickly
liikaa is an adverb meaning too much by itself, often used with verbs or nouns:
→ Hän työskentelee liikaa. = He/She works too much.
→ Minulla on liikaa töitä. = I have too much work.

So in this sentence:

liian stressaantunut is correct because stressaantunut is an adjective-like form (stressed), and we want liian before an adjective.

What exactly is stressaantunut? Is it a verb or an adjective?

Stressaantunut is the past participle of the verb stressaantua (to get stressed), and it’s used like an adjective:

Hän on stressaantunut. = He/She is stressed.
liian stressaantunut = too stressed.

In this sentence:

poikaystävä on liian stressaantunut
works structurally like “the boyfriend is too tired” – stressaantunut describes his state.

So grammatically, it’s a participle form of a verb functioning as a predicate adjective.

Why is töistä used here, and why is it in plural?

Työ = work, job. But Finnish very often uses the plural forms of työ to talk about “work” in the everyday/job sense:

töissä = “at work”
töihin = “to work” (going there)
töistä = “from work”

In the sentence:

liian stressaantunut töistä
literally: too stressed from (the) works → idiomatically: too stressed from work / because of work.

Why plural töistä and not singular työstä?

työstä is grammatically fine but tends to refer to a more specific/abstract “work” or “a job/task” in some contexts.
töistä is the normal idiomatic choice for everyday employment‑type work and for expressions like stressed from work.

So töistä (elative plural) here means from work in the sense of general job-related stress.