Við þynnum ungplönturnar.

• Við = we
• þynnum = the verb, we thin / we thin out
• ungplönturnar = the young plants or the seedlings, functioning as the direct object

So the structure is:

subject + verb + object

That is a very normal Icelandic word order in a main clause.

Þynnum is a conjugated verb form, while þynna is the infinitive.

The infinitive is the dictionary form:

• að þynna = to thin, to thin out

In this sentence, the subject is við (we), so the verb must be in the 1st person plural present tense:

• ég þynni = I thin
• við þynnum = we thin

The ending -um is the normal present-tense ending here for we.

In this context, að þynna means to thin out plants, especially in gardening.

That is, you remove some young plants so the remaining ones have more space, light, and nutrients.

So this is not about making something abstractly thinner only; it is a very natural verb for gardening contexts.

Because Icelandic often builds nouns by combining parts into a single compound word.

Ungplönturnar can be broken down like this:

• ung- = young
• plöntur = plants
• -nar = the (definite article, feminine plural form)

So:

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

This kind of compounding is extremely common in Icelandic.

It is the definite article attached to the noun.

In Icelandic, the is usually not a separate word. Instead, it is added to the end of the noun.

So:

• ungplöntur = young plants
• ungplönturnar = the young plants

Here -nar is the definite ending for this feminine plural form.

Because the sentence is referring to specific plants, not plants in general.

Compare:

• Við þynnum ungplöntur. = We thin young plants / seedlings.
  (more general, indefinite)
• Við þynnum ungplönturnar. = We thin the young plants / the seedlings.
  (specific ones)

So the version with -nar is definite.

That extra n is part of the definite form.

A simplified pattern is:

• indefinite plural: plöntur
• definite plural: plönturnar

When the article is added, Icelandic often inserts consonants or changes endings as part of normal declension patterns. So this is not random; it is part of how the noun behaves grammatically.

The key thing to learn is the full form:

• plöntur = plants
• plönturnar = the plants

And then in the compound:

• ungplöntur
• ungplönturnar

It is in the accusative plural because it is the direct object of the verb þynnum.

The verb að þynna takes a direct object, and that object appears in the accusative.

So in this sentence:

• Við = subject
• þynnum = verb
• ungplönturnar = accusative plural object

For learners, this is a very common Icelandic pattern: many verbs take an accusative object.

Usually, við is included.

Unlike some languages that regularly drop subject pronouns, Icelandic normally keeps them:

• Við þynnum ungplönturnar.

You may sometimes see subjects omitted in very informal contexts, diaries, instructions, or coordinated clauses, but as a normal full sentence, keeping við is the standard and safest choice.

Þ is pronounced like the th in thing, not like the th in this.

So:

• þ = voiceless th
• English comparison: thin, thing, thorn

That means þynnum begins with the same kind of sound as the start of thin.

A rough guide would be:

• Við ≈ vith
  The ð is a soft sound like th in this, though in connected speech it can be quite light.
• þynnum ≈ THIN-num
• ungplönturnar ≈ OONG-plurn-tur-nar
  This is only approximate, but it helps show the stress and general shape.

A few pronunciation notes:

• Icelandic stress is usually on the first syllable
• þ sounds like th in thing
• ö is not exactly an English sound; it is somewhat like the vowel in certain pronunciations of bird or burn, but shorter and clearer

So the sentence stress pattern is roughly:

VIÐ ÞYNNUM UNGplönturnar

Yes, but Við þynnum ungplönturnar is the most straightforward order.

Icelandic allows some flexibility, especially for emphasis. For example, another possible order might put something else first, but then the verb usually stays in second position in main clauses.

Still, for a learner, this sentence is best understood as the basic pattern:

subject + verb + object

That is the form you should be most comfortable producing first.