Learn Hardcore Finnish: Tasku on joskus niin täynnä papereita, etten löydä pankkikorttia. - The pocket is sometimes so full of papers that I can’t find my bank card.

Affirmative: Minä löydän pankkikortin.
- löydän = 1st person singular (I find)
Negative: Minä en löydä pankkikorttia.
- en = 1st person singular negative
- löydä = “connegative” form, no personal ending

In the sentence:

etten löydä = että
- en löydä → “that I don’t find”

So:

Positive: löydän
Negative: en löydä (main verb drops the -n)

Why is papereita in that form? Why not paperit or papereja?

The base word is paperi (a paper).

Plural forms:

Nominative plural: paperit
Partitive plural: papereita or papereja (both correct; papereita is more common in standard language)

Here we have papereita, partitive plural, because of the expression olla täynnä + partitive:

olla täynnä jotakin = to be full of something
So: täynnä papereita = full of papers (an indefinite amount)

If you said täynnä paperit, it would sound wrong; after täynnä, Finnish expects the partitive.

Why is pankkikorttia in the partitive case instead of pankkikortin?

The object pankkikorttia is partitive because the sentence is negative.

Compare:

Affirmative:
Löydän pankkikortin.
I find the bank card.
→ Total object: pankkikortin (genitive/accusative)
Negative:
En löydä pankkikorttia.
I do not find the bank card.
→ In most negative sentences, the object appears in the partitive: pankkikorttia

So in the full sentence:

etten löydä pankkikorttia
= että en löydä pankkikorttia
= that I don’t find (can’t find) the bank card.

This is a regular Finnish rule: negation tends to push the object into partitive.

Why does the Finnish sentence not show “my” (as in my bank card)?

Finnish often omits possessive pronouns when the owner is obvious from context.

In English you must say my bank card, but in Finnish you can just say:

En löydä pankkikorttia.
Literally: I don’t find (the) bank card.

Because I am the subject, it is normally understood that the card is mine.

If you really want to mark possession more explicitly, you can say:

En löydä pankkikorttiani. = I can’t find my bank card.
(-ni is a possessive suffix: my)

However, in everyday language, pankkikorttia without minun / -ni is completely natural and usually understood as “my bank card” here.

What exactly is täynnä? Is it an adjective like täysi?

The base adjective is:

täysi = full

But täynnä is a special adverbial form that behaves almost like a postposition:

olla täynnä + partitive = to be full of something

Examples:

Lasi on täynnä vettä. = The glass is full of water.
Tasku on täynnä papereita. = The pocket is full of papers.

Notice:

täynnä does not change for number or case:
- Lasi on täynnä. / Lasit ovat täynnä.
It always takes its “content” in the partitive: täynnä vettä, täynnä ihmisiä, täynnä papereita

Why is it Tasku on joskus niin täynnä… and not Taskussa on joskus…?

Both are possible, but they have different structures and emphasis.

Tasku on joskus niin täynnä papereita…
- Subject: tasku (pocket)
- Predicate: on … täynnä papereita (is full of papers)
- Focus: the state of the pocket (the pocket itself is sometimes so full)
Taskussa on joskus niin paljon papereita, etten löydä pankkikorttia.
- Locative subject-like phrase: taskussa (in the pocket)
- Existential structure: In the pocket there are sometimes so many papers…
- Focus: what is in the pocket, not the pocket as an entity

The original sentence uses the “X on täynnä Y” pattern: “X is full of Y”.

Can I move joskus to another position, like in English?

Yes, joskus (sometimes) is quite flexible in word order, and the meaning stays basically the same, with only small emphasis differences.

Possible placements:

Joskus tasku on niin täynnä papereita, etten löydä pankkikorttia.
Tasku on joskus niin täynnä papereita, etten löydä pankkikorttia. (original)
Tasku on niin täynnä papereita joskus, etten löydä pankkikorttia. (less usual, end-focus)

The most natural in standard style are:

Joskus tasku on…
Tasku on joskus…

Placing joskus at the beginning often gives it a bit more emphasis: Sometimes, the pocket is so full…

Is että en löydä always contracted to etten löydä? Are both forms correct?

Both forms are correct:

että en löydä pankkikorttia
etten löydä pankkikorttia

They mean the same thing: that I don’t find / can’t find the bank card.

The one-word forms are very common and fully standard:

etten, ettet, ettei, ettemme, ettette, etteivät

Using the separated että + (negative verb) is also correct and sometimes feels a bit more formal or careful. In spoken language, you will hear both.

Why is there no article like “the” or “a” before tasku and pankkikorttia?

Finnish simply does not have articles (no a/an or the). Definiteness or indefiniteness comes from context, word order, and case, not from separate words.

So:

Tasku on joskus niin täynnä papereita, etten löydä pankkikorttia.

can translate as:

The pocket is sometimes so full of papers that I can’t find my bank card.

or in another context maybe:

A pocket is sometimes so full of papers that I can’t find a bank card.

You have to choose the best English articles based on what makes sense in the situation.

Could I say Tasku on joskus niin täynnä papereita, etten löydä sitä instead of pankkikorttia?

You could, but the meaning would change slightly:

etten löydä pankkikorttia
→ repeats the noun, clear and neutral: that I can’t find the bank card.
etten löydä sitä (sitä = “it”, partitive)
→ refers back to something already mentioned, assumes the listener knows which it you mean.

In a short sentence like this, repeating pankkikorttia is very natural and clear in Finnish. Using sitä would be natural if you had already been talking about the card and wanted to avoid repetition.