kaupa ("to buy") is a high-frequency weak Class-2 verb with one memorable twist: its past tense changes the stem vowel from au to ey and adds a voiceless -ti ending — kaupi → keypti. This au → ey swap is not random; it is an old i-umlaut, the same change you see in hlaupa → hljóp's relatives and in the noun pair kaup "purchase" / keyptur "bought." The present is perfectly regular, so once you have memorised the single fact "the past of kaupa is keypti," the whole verb falls into place. This page also covers the everyday benefactive kaupa sér "buy oneself (something)."
Conjugation
Class: weak, Class 2 (the -ti / -di preterite), irregular au → ey stem vowel in the past. Auxiliary: hafa — ég hef keypt "I have bought."
| Principal parts | |
|---|---|
| Infinitive | að kaupa |
| 3sg present | kaupir |
| 3sg past | keypti |
| Supine | keypt |
| Person | Present (nútíð) | Past (þátíð) |
|---|---|---|
| ég | kaupi | keypti |
| þú | kaupir | keyptir |
| hann / hún / það | kaupir | keypti |
| við | kaupum | keyptum |
| þið | kaupið | keyptuð |
| þeir / þær / þau | kaupa | keyptu |
| Person | Present subjunctive | Past subjunctive |
|---|---|---|
| ég | kaupi | keypti |
| þú | kaupir | keyptir |
| hann / hún / það | kaupi | keypti |
| við | kaupum | keyptum |
| þið | kaupið | keyptuð |
| þeir / þær / þau | kaupi | keyptu |
| Non-finite & imperative | |
|---|---|
| Imperative (þú) | kauptu |
| Imperative (þið) | kaupið! |
| Supine | keypt |
| Past participle (m/f/n) | keyptur / keypt / keypt |
| Middle voice (miðmynd) | kaupast — "to be bought / be bought up" |
kaupa — buying things (+ accusative)
The thing bought stands in the accusative. This is the verb of every shop, market, and online order.
Ég kaupi alltaf kaffi á leiðinni í vinnuna.
I always buy coffee on the way to work.
Við keyptum nýjan ísskáp í síðustu viku.
We bought a new fridge last week.
Hvar keyptir þú þessa skó?
Where did you buy those shoes?
kaupa sér — "buy oneself (something)"
A very common pattern is the benefactive reflexive kaupa sér + accusative — "to buy oneself something." The reflexive pronoun sér is dative ("for oneself") and the purchased item is accusative. Icelandic uses this far more readily than English uses "buy oneself."
Hún keypti sér nýjan síma um daginn.
She bought herself a new phone the other day.
Ætlarðu að kaupa þér eitthvað að borða?
Are you going to buy yourself something to eat?
The imperative: kauptu
The command kauptu ("buy!") attaches -tu (devoiced from -ðu after the p). You will hear it in ads and casual encouragement.
Kauptu þér miða áður en það selst upp!
Buy yourself a ticket before it sells out!
kaupa vs. selja — buy vs. sell
The natural opposite is selja ("to sell"), a Class-1 weak verb with the past seldi. Keep the two apart: kaupa takes money out of your pocket, selja brings it in.
Þau keyptu húsið árið 2010 og seldu það í fyrra.
They bought the house in 2010 and sold it last year.
Common Mistakes
❌ Ég kaupaði bók í gær.
Incorrect — kaupa is not a Class-1 -aði verb; the past is keypti with au→ey
✅ Ég keypti bók í gær.
I bought a book yesterday.
❌ Hún keyptaði sér nýjan bíl.
Incorrect — double error: the past stem is already keypt-, so just add -i: keypti
✅ Hún keypti sér nýjan bíl.
She bought herself a new car.
❌ Við köypum mat á morgun.
Incorrect — the present has no umlaut; the stem stays kaup-, so it's kaupum
✅ Við kaupum mat á morgun.
We'll buy food tomorrow.
❌ Ég hef keyptur þennan bíl.
Incorrect — the perfect uses the invariable supine keypt, not the agreeing participle keyptur
✅ Ég hef keypt þennan bíl.
I have bought this car.
Key Takeaways
- kaupa / kaupir / keypti / keypt — weak Class 2; the present stem is kaup-, the past switches to keypt-.
- The signature change is au → ey in the past plus a voiceless -t-: keypti, keyptum, keyptu.
- There is no u-umlaut (the vowel is au/ey, not a): present "we" is plain kaupum.
- kaupa sér
- accusative = "buy oneself (something)" — far more common than the English equivalent.
- The opposite is selja ("sell," past seldi); the imperative is kauptu; auxiliary is hafa (ég hef keypt).
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Start learning Icelandic→Related Topics
- The Weak Preterite: -aði, -di, -ði, -tiA2 — How to choose and form the weak past tense — Class-1 -a verbs take -aði (tala → talaði, plural töluðum), Class-2 verbs take the short dental -di/-ði/-ti picked by the preceding sound (reyndi, dæmdi, keypti) — with the full tala paradigm and the 'when in doubt, -aði' default for unknown verbs.
- selja (to sell)A2 — Full conjugation of the weak j-verb selja (sel / seldi / seldu / selt), its double-object syntax (selja someone[dat] something[acc]), the middle voice seljast ('sell, be sold'), and the contrast with kaupa ('buy').