Learn Hardcore Finnish: Pidän pankkikorttia taskussa, koska lompakko unohtui kotiin. - I keep the bank card in my pocket because the wallet was forgotten at home.

Because the verb pitää (“to keep/hold”, also “to like”) typically takes its object in the partitive: pidän pankkikorttia = “I keep a/the bank card (on me)”.
With pitää, the partitive often feels like “having/keeping something as an ongoing state” rather than a completed, fully “affected” action.

Does pidän mean “I like” here? I thought pitää = “to like”.

Pitää can mean both “to like” and “to keep/hold”, and context decides.

Pidän kahvista. = “I like coffee.” (note the -sta/-stä “from/of” construction)
Pidän pankkikorttia taskussa. = “I keep the bank card in my pocket.” (object in partitive + location)

So this sentence uses the “keep/hold” meaning, not “like”.

Why is it taskussa and not tasku or something else?

Taskussa is the inessive case (“in”): tasku (pocket) + -ssa = “in the pocket”.
Finnish uses location cases instead of prepositions:

taskussa = in the pocket
taskusta = out of the pocket (elative)
taskuun = into the pocket (illative)

Why isn’t it taskussani (“in my pocket”)? Where is “my”?

Finnish often leaves possession implied if it’s obvious from context. Since the subject is “I” (pidän), the pocket is usually understood to be “my” pocket.
If you want to be explicit, you can say:

Pidän pankkikorttia taskussani. = “I keep the bank card in my pocket.”

What’s the role of koska here, and does it always mean “because”?

Koska introduces a reason clause: …, koska … = “…, because …”.
It can also mean “since” in some contexts. In this sentence it’s clearly causal: the reason for keeping the card in the pocket is that the wallet was forgotten at home.

Why is it lompakko unohtui and not unohdin lompakon (“I forgot the wallet”)?

Both are possible, but they express the idea differently:

Lompakko unohtui kotiin. literally “The wallet got forgotten at home.” (an “accidental / it happened” style; the wallet is the grammatical subject)
Unohdin lompakon kotiin. “I forgot the wallet at home.” (more direct; “I” is the subject)

Finnish often uses the “it got forgotten” type with unohtua, especially when the forgetting is presented as unintentional.

Why is unohtui in that form? What tense/person is it?

Unohtui is the past tense (imperfect), 3rd person singular of unohtua (“to be forgotten”).

present: unohtuu = “gets forgotten”
past: unohtui = “got forgotten / was forgotten”

Even if you mean “I forgot”, Finnish can still use the 3rd person here because the wallet is the subject.

Why is it kotiin (into home) instead of kotona/kodissa (at home/in the house)?

Kotiin is the illative case (movement “to/into”): it marks the place where something was left/ended up.
Unohtua often pairs naturally with an illative destination-type location:

Lompakko unohtui kotiin. = “The wallet was forgotten at home (left behind at home).”

Kotona (“at home”) would describe being at home, but it’s less natural for “left behind” in this exact construction.

Why does koti become kotiin with -iin? Is that a special rule?

Yes—some nouns form the illative with a doubled vowel + -n. Koti is one of them:

koti → kotiin (“into/to home”)

Other examples with a similar pattern:

maa → maahan (not doubled vowel, but another common illative pattern)
tie → tiehen
Illative formation varies by word type, so it’s often learned with patterns.

Could I change the word order? Is Finnish word order flexible here?

Fairly flexible, but changes emphasis. Neutral:

Pidän pankkikorttia taskussa, koska lompakko unohtui kotiin.

Possible variations:

Koska lompakko unohtui kotiin, pidän pankkikorttia taskussa. (emphasizes the reason first)
Pidän taskussa pankkikorttia, koska lompakko unohtui kotiin. (puts focus on “in the pocket”)

The basic meaning stays the same, but the “spotlight” shifts.

Why is there no “the/a” in Finnish? How do I know if it’s “a wallet” or “the wallet”?

Finnish has no articles, so lompakko can mean “a wallet” or “the wallet” depending on context. In a real situation it’s usually understood as “my/the wallet” because it’s something specific and known in the conversation.

Is Pidän pankkikorttia taskussa a natural way to say “I keep my bank card in my pocket”?

Yes, it’s understandable and natural. Depending on context, Finns might also say:

Pidän pankkikorttia taskussa mukana. (“…with me”, emphasizing carrying it)
Minulla on pankkikortti taskussa. (“I have a bank card in my pocket.”)
The sentence you have is perfectly fine, especially when explaining what you’re doing because the wallet isn’t with you.