Ég sé litla rót undir moldinni þegar ég færi plöntuna.

Because sjá is the dictionary form (the infinitive), meaning to see.

In the sentence, you need the 1st person singular present tense: I see.
The verb is irregular:

• að sjá = to see
• ég sé = I see
• þú sérð = you see
• hann/hún/það sér = he/she/it sees

So Ég sé ... is the correct finite verb form.

Because rót is the direct object of sé, and sjá takes the accusative case.

• lítil rót = a small root (nominative)
• litla rót = a small root (accusative)

The adjective lítill changes form to match the noun’s:

• gender: rót is feminine
• number: singular
• case: accusative

So:

• nominative: lítil rót
• accusative: litla rót

The noun rót itself happens to look the same in nominative and accusative singular, but the adjective shows the case clearly.

The base form is lítill, meaning small.

This adjective is irregular, so its forms are worth learning carefully. Relevant forms here are:

• masculine nominative singular: lítill
• feminine nominative singular: lítil
• feminine accusative singular: litla

Since rót is feminine and accusative singular, you get litla.

Because undir can take two different cases, depending on meaning:

• accusative for motion toward/to a position under something
• dative for location, when something is already under something

Here the root is located under the soil, so Icelandic uses the dative:

• undir moldinni = under the soil

If you said undir moldina, it would suggest movement to underneath the soil.

Moldinni is the dative singular definite form of mold.

Breakdown:

• mold = soil, earth
• moldin = the soil (nominative)
• moldinni = to/in/under the soil, depending on the preposition and case use

In this sentence, it is definite and dative because of undir used for location.

Because plöntuna is the direct object of færi, and færa takes the accusative case.

The noun is:

• planta = plant
• plöntu = accusative singular indefinite
• plöntuna = accusative singular definite, the plant

So ég færi plöntuna means I move the plant.

Here færi comes from að færa, which means to move, shift, or transfer something.

So:

• ég færi plöntuna = I move the plant

This is the 1st person singular present tense.

Be careful not to confuse it with other similar-looking forms in Icelandic. In this sentence, it is clearly the verb færa.

Because in Icelandic, many 1st person singular present tense forms end in -i.

For example:

• ég sé = I see
• ég færi = I move
• ég heyri = I hear

So to an English speaker, these forms may look unusual at first, but they are completely normal Icelandic present-tense forms.

Þegar means when and introduces a time clause.

So the sentence is structured like this:

• Ég sé litla rót undir moldinni = I see a small root under the soil
• þegar ég færi plöntuna = when I move the plant

Together: I see a small root under the soil when I move the plant.

Yes. You can put the þegar clause first:

• Þegar ég færi plöntuna, sé ég litla rót undir moldinni.

That is also correct.

A useful point: Icelandic is a V2 language, which means the finite verb usually comes in the second position in a main clause. So when the time clause comes first, the main clause becomes:

• sé ég ... not
• ég sé ...

Because Icelandic usually puts the definite article onto the end of the noun instead of using a separate word.

Examples from the sentence:

• moldinni = the soil (with case ending too)
• plöntuna = the plant

By contrast, litla rót is indefinite, so it means a small root, not the small root.

Here are the main dictionary forms:

• ég = I
• sjá = to see
• lítill = small
• rót = root
• undir = under
• mold = soil, earth
• þegar = when
• færa = to move, shift
• planta = plant

This is useful because many of the forms in the sentence are inflected, so what you see is not always the dictionary form.