Plantan dó í vetur.

Because -n here is the definite article attached to the noun.

• planta = a plant / plant
• plantan = the plant

In Icelandic, the is very often added to the end of the noun instead of being a separate word. Since planta is a feminine noun ending in -a, the definite nominative singular form is plantan.

Plantan is in the nominative singular.

That is because it is the subject of the sentence — it is the thing doing or undergoing the action of the verb:

• Plantan = the subject
• dó = died

So the sentence structure is basically The plant died ...

Dó is the past tense of deyja, which means to die.

So:

• deyja = to die
• dó = died

It looks very different because deyja is an irregular (strong) verb. This is similar to English verbs like:

• go → went
• die → died (less dramatic in English, but still a change)

In Icelandic, strong verbs often change their vowel in the past tense, so deyja → dó is something you need to learn as a set.

Because Icelandic can express this with a simple past tense verb all by itself.

English says:

• The plant died

Icelandic does the same thing:

• Plantan dó

No extra helper verb is needed. The past meaning is already built into dó.

Í vetur is a common time expression meaning roughly this winter or over the winter, depending on context.

Even though í often means in, this phrase is best learned as a whole expression. It usually refers to one specific winter period, not winter in general.

With a past-tense verb like dó, it often means something like:

• this past winter
• over the winter

The exact English wording depends on the situation and on when the speaker is talking.

Because í vetur is a very common set time expression in Icelandic.

In these seasonal expressions, Icelandic often uses the season word without the definite article:

• í vetur = this winter / over the winter
• í sumar = this summer
• í haust = this autumn
• í vor = this spring

If you said í vetrinum, that would sound more like in the winter referring to a more specifically defined winter period. For learners, it is best to memorize í vetur as the normal expression here.

Here vetur is in the accusative singular.

The preposition í can take different cases in different situations, and with time expressions like í vetur, the form is best treated as a standard idiomatic pattern.

So even if you already know the basic rule that í can go with dative or accusative, in this sentence the most useful thing is simply to learn:

• í vetur = a fixed time expression

Yes, that is possible.

• Plantan dó í vetur = neutral word order
• Í vetur dó plantan = puts more emphasis on this winter

Icelandic is a V2 language, which means the finite verb usually stays in the second position of the clause. So if you move Í vetur to the front, dó still comes next:

• Í vetur
  • dó
    • plantan

Both versions are grammatical; the difference is mainly emphasis and style.