Vatnið eyðilagði efnið.

It is the definite article, so it corresponds to English the.

• vatn = indefinite noun
• vatnið = the water
• efni = indefinite noun
• efnið = the material/substance

In Icelandic, the is usually added to the end of the noun instead of being a separate word. The exact ending changes with gender, number, and case. Here, both nouns are neuter singular, so you get -ið.

The dictionary forms are:

• vatn
• efni

If you remove the suffixed article -ið, you get the basic form you would normally look up in a dictionary.

Both nouns are neuter.

That matters because in Icelandic, gender affects:

• the form of the definite article
• adjective endings
• pronouns
• some case forms

In this sentence, the neuter gender helps explain why both definite forms end in -ið.
However, the verb does not change for gender here.

• Vatnið is the subject, so it is in the nominative
• efnið is the direct object, so it is in the accusative

A useful thing to notice is that for many neuter singular nouns, the nominative and accusative look the same. So even though the cases are different in function, the forms here are identical on the surface.

In this sentence, word order is the main clue.

• Vatnið comes before the verb, so it is understood as the subject
• efnið comes after the verb, so it is understood as the object

Because nominative and accusative look the same here, the sentence would become confusing if you relied only on endings. In practice, Icelandic often uses case endings to show who does what, but in a sentence like this, word order and context do a lot of the work.

The infinitive is eyðileggja.

eyðilagði is the 3rd person singular past tense form.

A helpful set of forms to learn is:

• eyðileggja — infinitive
• eyðileggur — present singular
• eyðilagði — past singular
• eyðilagt — past participle/supine form

So this is not something you get by just mechanically attaching one ending to the infinitive. The stem changes, so it is best learned as part of the verb’s pattern.

No. In this kind of form, the verb agrees with the subject in person and number, not gender.

So eyðilagði is used because the subject is 3rd person singular.
It would be the same past-tense singular form with:

• a masculine noun
• a feminine noun
• a neuter noun

Gender does not change the verb form here.

Usually, ð is pronounced roughly like the th in this.

A few useful points:

• it is a real letter in Icelandic
• it does not occur at the beginning of words
• in connected speech, especially in endings, it can sound softer than an English speaker expects

Also, Icelandic stress normally falls on the first syllable of each word, so learners should say:

• VAT-nið
• EY-ði-lag-ði
• EF-nið

You can change word order in Icelandic in some contexts, but with this exact sentence you need to be careful.

If you simply switch the nouns:

• Efnið eyðilagði vatnið

that will normally be understood as the opposite meaning, because now efnið looks like the subject.

So although Icelandic word order can be flexible, this sentence is one where the normal order strongly helps signal who is doing the action.