Sláttuvélin virkar betur þegar grasflötin er þurr.

Sláttuvélin is a single noun made of:

• sláttuvél = lawn mower
• -in = the suffixed definite article, so the

So sláttuvélin means the lawn mower.

A useful thing to notice is that Icelandic often builds nouns into compounds. Here the idea is basically mowing machine.

Because Icelandic usually puts the onto the end of the noun instead of using a separate word in front.

So:

• sláttuvél = lawn mower
• sláttuvélin = the lawn mower

and:

• grasflöt = lawn / grass lawn
• grasflötin = the lawn

This -in ending here is the definite article attached to a feminine singular noun in the nominative.

Both are singular nominative.

• sláttuvélin is the subject of virkar
• grasflötin is the subject of er þurr

So the structure is:

• Sláttuvélin virkar betur = the lawn mower works better
• þegar grasflötin er þurr = when the lawn is dry

Since both nouns are subjects, nominative is exactly what you would expect.

Virkar is the present tense, 3rd person singular of virka.

Here it means:

• works
• functions
• operates

For a machine, virka is a very natural verb. So sláttuvélin virkar means the lawn mower works / functions.

Because betur is an adverb, while betri is an adjective.

In this sentence, betur modifies the verb virkar:

• virkar betur = works better

You are describing how it works, not describing the mower itself.

Compare:

• Hún virkar betur. = It works better. → adverb betur
• Hún er betri. = It is better. → adjective betri

A very useful pair to remember is:

• góður = good
• vel = well
• betri = better, as an adjective
• betur = better, as an adverb

Here þegar means when.

It introduces a time clause:

• þegar grasflötin er þurr = when the lawn is dry

One thing that can confuse learners is that þegar can also mean already in other contexts. But in this sentence it is clearly the conjunction when, because it is followed by a full clause.

Because þurr has to agree with grasflötin, which is feminine singular nominative.

In Icelandic, predicate adjectives usually agree with the noun they describe. So in:

• grasflötin er þurr

the adjective matches grasflötin.

For þurr, the nominative singular forms are:

• masculine: þurr
• feminine: þurr
• neuter: þurrt

So here the correct form is þurr because grasflötin is feminine singular.

Er is the present tense of vera, the verb to be.

So:

• grasflötin er þurr = the lawn is dry

It is just the normal copula, linking the subject grasflötin with the adjective þurr.

Yes. You can say:

Þegar grasflötin er þurr, virkar sláttuvélin betur.

That still means the same thing.

But notice the word order in the main clause after the introductory clause:

• virkar sláttuvélin betur

This happens because Icelandic is generally a verb-second language in main clauses. When the þegar clause comes first, the finite verb of the main clause usually comes next.

Because inside this subordinate clause, the order is very normal:

• þegar = when
• grasflötin = the lawn
• er = is
• þurr = dry

So it is basically:

when + subject + verb + adjective

That is a very common and useful clause pattern in Icelandic.

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

So þurr starts with that voiceless th sound.

A rough guide:

• þurr ≈ thur with the th of think

This letter is called thorn, and it is very common in Icelandic.