Learn Hardcore Finnish: Tämä reppu on kevyempi kuin minun laukkuni.

In the sentence Tämä reppu on kevyempi kuin minun laukkuni, tämä points to a backpack that is close to the speaker (for example, the one you’re holding or touching).

What is the difference between reppu and laukku?

Both can be translated as bag, but they refer to different kinds of bags:

reppu = backpack / rucksack (a bag you carry on your back, with two shoulder straps)
laukku = bag in a more general sense (handbag, briefcase, suitcase, shoulder bag, etc., depending on context)

So the sentence literally says: “This backpack is lighter than my bag.”

Why is the verb on used here, and does it agree with reppu?

On is the 3rd person singular form of the verb olla (to be).

olla = to be
(hän / se) on = he / she / it is

In Tämä reppu on kevyempi…, the subject is tämä reppu (this backpack), which is singular, so you use on.

Finnish verbs agree with the person and number of the subject:

minä olen (I am)
sinä olet (you are, singular)
hän / se on (he / she / it is)
me olemme (we are)
te olette (you are, plural or polite)
he ovat (they are)

What does kevyempi mean, and how is this form built?

Kevyt means light (in weight).
Kevyempi is the comparative form: lighter.

Finnish forms the comparative of adjectives mostly with the ending -mpi:

kevyt → kevyempi (light → lighter)
suuri → suurempi (big → bigger)
halpa → halvempi (cheap → cheaper)
pitkä → pidempi (long → longer)

For kevyt:

The stem used is kevy-.
Then you add -empi → kevyempi.

The next degree (superlative) of kevyt is kevyin = lightest.

What does kuin mean, and when do you use it?

Kuin here means than, used in comparisons with the comparative (-mpi) form:

kevyempi kuin = lighter than
suurempi kuin = bigger than
nopeampi kuin = faster than

So Tämä reppu on kevyempi kuin minun laukkuni = This backpack is lighter than my bag.

Be careful not to confuse kuin with kuten:

kuin = than (after comparatives), also as in structures like niin … kuin …
kuten = as, like (in the sense of “in the way that”)

For ordinary “X is …er than Y” comparisons, you need kuin.

What does the ending -ni in laukku → laukkuni mean?

The ending -ni is a possessive suffix meaning my.

laukku = a / the bag
laukkuni = my bag

Other common possessive suffixes are:

-ni = my
-si = your (singular)
-nsa / -nsä = his / her / its / their
-mme = our
-nne = your (plural)

So minun laukkuni literally contains “my” twice:

minun = my (genitive of minä, I)
laukkuni = my bag (bag + my)

This “double marking” is very common and natural in modern Finnish.

Why do we say minun laukkuni and not just minun laukku?

In standard Finnish, a possessed noun is normally marked either with:

Just the possessive suffix:
- laukkuni = my bag
  (This alone is fully correct and common in written language.)
The genitive pronoun plus the possessive suffix:
- minun laukkuni = my bag
  (Very common and natural in both spoken and written Finnish.)

Using only the genitive pronoun without the suffix (e.g. minun laukku) is typical in spoken, colloquial Finnish, but is not considered fully standard in formal writing.

So in a textbook-style sentence, minun laukkuni is exactly what you’re expected to see.

Can I leave out minun and just say Tämä reppu on kevyempi kuin laukkuni?

Yes.

Tämä reppu on kevyempi kuin laukkuni.
= This backpack is lighter than my bag.

This version is fully grammatical and maybe a bit more typical in written Finnish, because the possessive suffix -ni alone already expresses “my”.

Including minun adds a bit of emphasis or clarity, similar to stressing my in English:

Tämä reppu on kevyempi kuin minun laukkuni.
→ “This backpack is lighter than my bag (as opposed to someone else’s).”

What case is laukku / laukkuni in here, and why isn’t there any special “than-case”?

In Tämä reppu on kevyempi kuin minun laukkuni, the word laukkuni is in the nominative singular (the basic dictionary form).

In English, “than” introduces something that looks like an object, but in Finnish the structure after kuin is often just a normal noun phrase in nominative:

hän on pidempi kuin minä = he is taller than I
tämä reppu on kevyempi kuin laukkuni = this backpack is lighter than my bag

No special “than-case” exists. You just use kuin + normal form (most commonly nominative), unless another case is required by a different grammatical reason.

Could the word order be different, like Reppu tämä on kevyempi kuin minun laukkuni?

Finnish word order is freer than English, but it still has a neutral order.
Here, the neutral word order is:

Tämä reppu on kevyempi kuin minun laukkuni.
(Subject – Verb – Predicative – kuin-phrase)

Other orders are possible, but they change the style or focus. For example:

Tämä reppu on minun laukkuani kevyempi.
(A bit more emphatic / literary-sounding; note laukkua in partitive.)
Kevyempi kuin minun laukkuni on tämä reppu.
(Unusual and very emphatic; sounds poetic or marked.)

Reppu tämä on kevyempi… would sound awkward and unnatural in modern standard Finnish unless you are imitating some special rhetorical style or dialect.

Does kevyempi have to agree in number or case with reppu?

Yes, in principle adjectives in Finnish agree with the noun in number and case. In this particular sentence, both the subject reppu and the predicate adjective kevyempi are in the singular nominative, so you just see their base comparative form.

Examples showing agreement:

Yksi kevyt reppu = one light backpack (singular nominative)
Kaksi kevyttä reppua = two light backpacks (plural partitive)
Kevyemmässä repussa = in a lighter backpack (singular inessive)
Kevyemmissä repuissa = in lighter backpacks (plural inessive)

In Tämä reppu on kevyempi, there is no case ending on reppu in this role, so kevyempi is also nominative singular.

How would I say “This backpack is lighter than that one” in Finnish?

You can say:

Tämä reppu on kevyempi kuin tuo.
= This backpack is lighter than that (one).

If you want to keep the word laukku:

Tämä reppu on kevyempi kuin tuo laukku.
= This backpack is lighter than that bag.

Here:

tämä = this (near the speaker)
tuo = that (a bit further away, visible / over there)

How do you pronounce the tricky vowels and consonants in this sentence?

Approximate pronunciation (IPA):

tämä → [ˈtæmæ]
- ä like the a in cat, but usually a bit clearer and fronted.
reppu → [ˈrepːu]
- Double pp is a long consonant: hold the p slightly longer than in English.
kevyempi → [ˈkeʋyempi]
- y is a rounded front vowel, like the French u in lune or German ü in München.
- v is often pronounced between [v] and [ʋ], like a soft v/w.
kuin → [ˈkuin]
- ui is a diphthong, like English oo
  - ee blended (koo-in quickly).
minun → [ˈminun]
laukkuni → [ˈlaukːuni]
- au is a diphthong like ow in cow but a bit more distinct.
- Double kk is a long k.

Putting it together slowly: [ˈtæmæ ˈrepːu on ˈkeʋyempi ˈkuin ˈminun ˈlaukːuni].