Learn Hardcore Finnish: Ilman leimaa paperi ei ole voimassa. - Without a stamp, the paper is not valid.

leima is the dictionary form (nominative).
leimaa is the partitive singular, required by ilman.
leiman (genitive) is generally not used after ilman in modern standard Finnish (you may occasionally see genitive in older/poetic language, but the normal rule is partitive).

So ilman leimaa is simply the standard grammatical form.

How does the negation work in paperi ei ole voimassa?

Finnish negation uses a separate negative verb that conjugates for person/number:

ei = 3rd person singular (he/she/it does not)

After the negative verb, the main verb appears in the connegative form:

olla (to be) → ole

So:

(paperi) ei ole = is not

Compare:

paperi on voimassa (the paper is valid/in force)
paperi ei ole voimassa (the paper is not valid/in force)

Why is it paperi (nominative) even though the sentence is negative? Shouldn’t it be partitive?

Not every negative sentence forces a partitive subject. Here, paperi is the clear, definite subject of a normal copula sentence (X is not Y), so nominative is natural:

Paperi ei ole voimassa.

Partitive subjects are especially common in existential or “there is/are”-type sentences, or when the subject is indefinite/amount-like:

Täällä ei ole paperia. (There is no paper here.)

What exactly is voimassa grammatically?

Voimassa is the inessive case form of voima (power/force):

voima → voimassa = literally in force

In modern Finnish it’s very commonly used as part of the fixed expression:

olla voimassa = to be valid / to be in force

So voimassa functions like an adverbial complement meaning “in effect/valid.”

Is olla voimassa an idiom, and can I replace it with something else?

Yes, olla voimassa is a very common set phrase, especially for laws, rules, documents, tickets, permits, etc.

Depending on context, alternatives include:

ei päde (doesn’t apply / isn’t valid) — common but slightly different nuance
ei kelpaa (isn’t accepted) — more about acceptance than legal validity

But ei ole voimassa is often the safest for “not legally/officially valid.”

Can the word order change? Why is Ilman leimaa first?

Finnish word order is flexible. Putting Ilman leimaa first highlights the condition (“without a stamp…”).

You can also say:

Paperi ei ole voimassa ilman leimaa.

Both are correct; the difference is mostly emphasis and information flow.

Does ilman leimaa mean “without a stamp” in general, or “without the stamp” (a specific one)?

By itself, ilman leimaa is usually interpreted as without a stamp (at all) in the relevant sense. Finnish doesn’t have articles (a/the), so specificity comes from context.

If you want to emphasize “without any stamp whatsoever,” you can add:

Ilman mitään leimaa paperi ei ole voimassa. (Without any stamp, the paper is not valid.)

How would I say this in the past or future?

Past:

Ilman leimaa paperi ei ollut voimassa. (was not valid)

Future / “will not be valid” (common ways):

Ilman leimaa paperi ei ole voimassa. (often still used if it’s a general rule)
Ilman leimaa paperi ei tule voimaan. (will not enter into force / will not become valid)

What are the key pronunciation points in this sentence?

A few helpful points:

Stress is usually on the first syllable: IL-man LEI-maa PA-pe-ri ei O-le VOI-mas-sa
ei is pronounced like “ay” in say (roughly).
Double vowels are long: lei-maa has a long aa at the end.
voimassa has clear syllables: voi-mas-sa (with ss pronounced longer than a single s).