Learn Hardcore Finnish: Teidän reppunne on kevyt.

more than one person, or
one person in polite/formal address (the “V-form”). The capital T is just because it starts the sentence; otherwise it’s written teidän. Optional honorific capitalization (Te/Teidän) is used mainly in very formal letters.

Why do we have both Teidän and the -nne ending in reppunne? Isn’t that redundant?

They both mark the same possession. Using both is common in formal/written Finnish. In everyday Finnish, you can use either:

Suffix only: Reppunne on kevyt.
Genitive only: Teidän reppu on kevyt.
Both (more formal/explicit): Teidän reppunne on kevyt.

What does the suffix -nne in reppunne mean, and where does it go in a word?

-nne is the 2nd person plural possessive suffix “your.” It attaches after number/case endings: stem + number/case + -nne. Examples:

reppunne = reppu + -nne → “your backpack(s)”
repussanne = reppu + ssa + -nne → “in your backpack(s)”
reppuunne = reppu + (illative -Vn, with n replaced by the suffix) + -nne → “into your backpack(s)”

Is reppunne singular or plural?

By form it can mean either “your backpack” or “your backpacks.” Number is shown by the verb and the adjective:

Singular: Reppunne on kevyt. (“Your backpack is light.”)
Plural: Reppunne ovat kevyet. (“Your backpacks are light.”)

Why is the verb on singular here and not ovat?

Because the sentence is interpreted as singular (“your backpack”). The verb and predicate adjective agree with that reading: on kevyt.

Does the adjective kevyt have to agree with the subject?

Yes. In copula sentences, the predicative adjective agrees in number:

Singular: Reppu on kevyt.
Plural: Reput ovat kevyet. In the negative plural you’ll see partitive plural: Reput eivät ole kevyitä.

How do I say “Your backpacks are light”?

Several correct options:

Teidän reppunne ovat kevyet. (with suffix; somewhat formal)
Teidän reput ovat kevyet. (no suffix; common)
Reppunne ovat kevyet. (suffix only)

Can I say Teidän reppu on kevyt instead?

Yes. That’s very common in modern/speech-like Finnish. The possessive suffix is optional when a genitive possessor (Teidän) is present.

Can I drop Teidän and just say Reppunne on kevyt?

Yes, if it’s clear whose backpack you mean from context (e.g., you’re speaking to them). It sounds more formal/bookish than everyday speech.

Why isn’t it Teidän repun on kevyt?

Repun is genitive (“of a backpack”) and can’t be the subject here. With a possessive suffix, use the nominative for a subject: Teidän reppunne on kevyt. Use the genitive when another noun follows:

Teidän repunne hihna = “the strap of your backpack” (repun + -nne = repunne; note pp → p consonant gradation)

How would I say this to one person informally?

Sinun reppusi on kevyt. In colloquial speech: Sun reppu on kevyt.

How do I ask “Is your backpack light?”

Onko reppunne kevyt? Alternatives: Onko teidän reppu kevyt? (no suffix) / Onko reppusi kevyt? (informal singular)

Do I need articles (a/the) in Finnish?

No. Finnish has no articles. Context handles definiteness, so Teidän reppunne simply means “your backpack.”

How is Teidän reppunne on kevyt pronounced?

Stress the first syllable of each word: TEI-dän REP-pun-ne on KE-vyt.
pp and nn are long consonants—hold them longer.
y is a front rounded vowel [y], like French u in “lune.”
ä is a front vowel [æ], like the a in “cat.”

Why does Teidän have ä in it?

It’s the standard genitive form of te: teidän. Several Finnish pronoun genitives use ä (minun, sinun, hänen, meidän, teidän, heidän). It’s not “teidan.”

Is Teidän reppunne on kevyt natural in everyday speech?

It’s correct but sounds somewhat formal/polite (especially if addressing one person). In casual speech people more often say:

Sinun reppusi on kevyt. / Sun reppu on kevyt. (to one person)
Teidän reput ovat kevyet. (to several people, plural)