Learn Hardcore Finnish: Unohdin taas käyttäjätunnuksen, joten minun piti vaihtaa salasana. - I forgot the username again, so I had to change the password.

Unohdan = I forget / I will forget (present)
Unohdin = I forgot (past)
Olen unohtanut = I have forgotten (present perfect, literally I am forgotten)

What is the role of taas here, and where can it go in the sentence?

Taas means again.

In Unohdin taas käyttäjätunnuksen, it modifies the whole action forgetting the username, stressing that this has happened before.

You can change its place somewhat:

Taas unohdin käyttäjätunnuksen. – Again I forgot the username. (Emphasis on again)
Unohdin käyttäjätunnuksen taas. – Also possible, but less common in this kind of sentence; it can sound slightly more like an afterthought.

The most neutral place is usually right after the verb or right before it: Unohdin taas… or Taas unohdin…

Why does käyttäjätunnuksen end in -n?

Käyttäjätunnuksen is in the genitive/accusative form. Here it functions as a total object of the verb unohtaa.

Basic form (nominative): käyttäjätunnus – a username / the username (dictionary form)
Genitive/accusative: käyttäjätunnuksen – the username as a complete object of a completed action

With a completed, one‑time past action, the object is often in this -n form:

Unohdin käyttäjätunnuksen. – I forgot the username.
Ostin auton. – I bought a car (a specific car).

So the -n marks it as the object of forgetting.

Could you say Unohdin käyttäjätunnukseni instead of käyttäjätunnuksen? What’s the difference?

Yes, you can say Unohdin käyttäjätunnukseni.

Unohdin käyttäjätunnuksen.
→ I forgot the username (context will usually make it clear it’s my username).
Unohdin käyttäjätunnukseni.
→ I forgot my username (the possessive suffix -ni explicitly shows my).

Both are natural. Finnish often leaves possession implicit if the context is obvious, so käyttäjätunnuksen is very common in situations where everybody knows you’re talking about your own username.

What does joten mean, and is it similar to so in English?

Yes, joten corresponds closely to English so / therefore.

Unohdin taas käyttäjätunnuksen, joten minun piti vaihtaa salasana.
→ I forgot the username again, so I had to change the password.

Joten introduces a logical consequence of what was said before. It is:

Neutral, standard written and spoken Finnish
Similar to siksi (for that reason) or niin että (so that), but:
- siksi is usually adverbial: Unohdin tunnuksen, ja siksi vaihdoin salasanan.
- niin että is more like in such a way that / so that and is often used differently.

In causal “A happened, so B happened” sentences, joten is a very typical choice.

What is minun piti literally, and why is it used to say “I had to”?

Minun piti comes from the verb pitää in the sense “to have to / must”.

pitää (here) = to have to, must
piti = past tense, 3rd person singular
minun = my (genitive; used to mark the “experiencer” of necessity)

So literally minun piti vaihtaa salasana is something like:

it had to be (for) me to change the password → I had to change the password.

This is a very common way to express obligation/necessity:

Minun pitää lähteä. – I have to go.
Minun piti lähteä. – I had to go.

Other close alternatives:

Minun täytyy vaihtaa salasana. – I must / I have to change the password.
Minun täytyi vaihtaa salasana. – I had to change the password.

All are natural; pitää and täytyä are both frequent for “must / have to”.

Do you have to say minun, or can you just say piti vaihtaa salasana?

You can omit minun if it’s clear from context who is involved:

Piti vaihtaa salasana. – (I / we / someone) had to change the password.

In everyday speech, people often drop the genitive pronoun (minun, sinun, etc.) when it’s obvious who they mean.

However, including minun makes it clear and explicit that I was the one who had to do it:

Minun piti vaihtaa salasana. – specifically I had to change it.

For learners, it’s safer to include the pronoun (minun piti…) until you’re comfortable with how Finnish speakers use context.

Why is salasana not in the -n form like käyttäjätunnuksen? Why not salasanan?

Good observation: käyttäjätunnuksen has -n, but salasana does not.

Here’s why:

Käyttäjätunnuksen is the object of a finite past tense verb (Unohdin). In that case, a completed action with a total object often uses the genitive/accusative (-n).
Salasana is the object of an infinitive verb, vaihtaa, in the phrase minun piti vaihtaa salasana.

With täytyy/pitää + infinitive, the object of the infinitive usually appears in the basic (nominative) form when it’s about doing the action in general, not about a clearly delimited, already‑done event:

Minun pitää ostaa auto. – I have to buy a car. (object in basic form)
Ostin auton. – I bought the car/a car. (object in -n form)

So salasana here is in the nominative as the object of the infinitive vaihtaa. It’s like saying “I had to change (a/the) password” as a task, not describing a completed past event with a finite verb.

What does käyttäjätunnus literally mean?

Käyttäjätunnus is a compound word:

käyttäjä = user
tunnus = identifier, code, symbol, mark

So käyttäjätunnus literally means something like user identifier, which matches the English idea of username or user ID.

The -n in käyttäjätunnuksen is just the case ending; the base word is käyttäjätunnus.

Is the word order fixed? Could I say Taas unohdin käyttäjätunnuksen, joten piti vaihtaa salasana?

Yes, that sentence is natural Finnish and shows some common variations:

Taas unohdin käyttäjätunnuksen, joten piti vaihtaa salasana.
- Taas at the beginning adds emphasis to again.
- Dropping minun in piti vaihtaa salasana is normal in informal speech.

Word order in Finnish is more flexible than in English, and you can move parts around to change the emphasis slightly:

Unohdin taas käyttäjätunnuksen, joten minun piti vaihtaa salasana.
(neutral; focus on the act of forgetting)
Taas unohdin käyttäjätunnuksen, joten minun piti vaihtaa salasana.
(stronger emphasis on “again”)
Unohdin käyttäjätunnuksen taas, joten minun piti vaihtaa salasana.
(possible; “again” feels a bit like an afterthought)

All are grammatically correct; the differences are mostly about nuance and emphasis.