Eftir því sem radísurnar vaxa þynnum við ungplönturnar.

Eftir því sem is an idiomatic expression meaning as, in proportion as, or the more/as when something changes gradually.

So here:

Eftir því sem radísurnar vaxa ...
means As the radishes grow ...

It is better to learn eftir því sem as a fixed expression, rather than trying to translate each word separately.

This is because Icelandic normally follows a verb-second pattern in main clauses.

The first slot in the sentence is taken by the adverbial clause:

Eftir því sem radísurnar vaxa

After that, the finite verb of the main clause must come next:

þynnum

Then the subject comes:

við

So:

Eftir því sem radísurnar vaxa þynnum við ungplönturnar.

If the main clause stood alone, you would normally say:

Við þynnum ungplönturnar.

Þynnum is the present tense, 1st person plural form of þynna, which means to thin or to thin out.

So:

• þynna = to thin out
• þynnum = we thin / we are thinning

In gardening English, thin out is often used, but Icelandic does not need a separate word for out here. The verb þynna already carries that meaning.

It comes after the verb because of the verb-second rule explained above.

As for leaving it out: in normal Icelandic, við is usually not omitted. Even though þynnum already shows we, Icelandic generally still uses the subject pronoun.

So þynnum við is normal and expected here.

Ungplönturnar is the definite plural of ungplanta, literally young plant.

In this context, it means the seedlings or the young plants.

So the sentence is talking about thinning the young radish plants as they grow, which is a normal gardening idea: you remove some seedlings so the others have more space.

• radísurnar is the subject of vaxa, so it is in the nominative
• ungplönturnar is the object of þynna, so it is in the accusative

A useful thing to notice is that for these nouns, the nominative plural definite and accusative plural definite look the same on the surface. So you identify the case mainly from the noun’s role in the sentence, not from a visibly different ending.

Because both are feminine plural definite forms.

In Icelandic, the definite article is usually attached to the end of the noun instead of being a separate word.

So:

• radísur = radishes
• radísurnar = the radishes

and

• ungplöntur = young plants / seedlings
• ungplönturnar = the young plants / the seedlings

So the ending -nar here is basically part of saying the in the plural.

This is a common Icelandic noun pattern.

The singular is:

• ungplanta

The indefinite plural is:

• ungplöntur

Then the definite plural is:

• ungplönturnar

Two things are happening:

1. many feminine nouns in -a form the plural with -ur
2. the stem vowel often changes from a to ö before that u sound; this is called u-umlaut

So:

• planta → plöntur
• therefore ungplanta → ungplöntur → ungplönturnar

This kind of vowel change is very common in Icelandic.

Usually it describes a general or repeated process.

In gardening instructions or descriptions, Icelandic often uses the present tense the same way English does:

As the radishes grow, we thin the seedlings.

That means this is what we do during that stage of growth, not necessarily only at one exact moment.

Icelandic often uses the definite form when talking about the specific plants under discussion, even in a general practical statement.

So radísurnar and ungplönturnar suggest:

• the radishes we are growing
• the seedlings in that bed or crop

English sometimes leaves out the in this kind of general instruction, but Icelandic often sounds more natural with the definite form when the situation is understood from context.