Í maí er ekki bara garðurinn grænni, heldur er líka auðveldara að sitja úti, og í júní borðum við stundum kvöldmat á svölunum.

This is standard Icelandic V2 word order (verb-second).

In a main clause, the finite verb usually comes in the second position. If you begin the sentence with something other than the subject, such as a time phrase like Í maí (In May), the verb still stays second:

• Í maí er ...
• not Í maí garðurinn er ...

So even though garðurinn is the subject, it comes after er because Í maí has been placed first.

This is a very common Icelandic structure meaning not only ..., but also ...

In your sentence:

• ekki bara garðurinn grænni
• heldur er líka auðveldara að sitja úti

This corresponds to:

• not only is the garden greener
• but it is also easier to sit outside

A few useful points:

• bara = only / just
• ekki bara = not only
• heldur in this structure = but rather / but

Often Icelandic also uses heldur líka or just heldur with líka elsewhere in the clause.

Because Icelandic often omits a repeated verb when it is understood from context.

A fuller version would be:

• Í maí er ekki bara garðurinn grænni, heldur er líka auðveldara ...

The first part can be understood as:

• Í maí er garðurinn ekki bara grænni ...

But Icelandic allows this compressed structure very naturally.

So garðurinn grænni is understood as garðurinn er grænni.

Garðurinn means the garden. The ending -inn is the definite article attached to the noun.

So:

• garður = garden
• garðurinn = the garden

Icelandic usually attaches the to the end of the noun instead of using a separate word like English the.

Grænni is the comparative form of grænn (green).

• grænn = green
• grænni = greener

Since the sentence is comparing how things are in May, Icelandic uses the comparative:

• garðurinn grænni = the garden greener
• understood as the garden is greener

Here grænni matches garðurinn, which is masculine singular nominative.

Auðveldara is the comparative of auðveldur (easy), and here it is used in a neuter singular impersonal construction.

• auðveldur = easy
• auðveldara = easier

Icelandic often uses neuter singular adjectives in general statements like:

• Það er auðvelt = It is easy
• Það er auðveldara = It is easier

So:

• er líka auðveldara að sitja úti = it is also easier to sit outside

Even though there is no written það here, English often translates it with it.

Here að marks the infinitive, like English to in to sit.

• sitja = sit
• að sitja = to sit

So:

• auðveldara að sitja úti = easier to sit outside

This is a very common pattern:

• gaman að lesa = fun to read
• gott að sofa = good to sleep
• erfitt að skilja = difficult to understand

Úti means outside / outdoors and describes location.

• sitja úti = sit outside

By contrast, út usually expresses motion outward:

• fara út = go out

So the difference is similar to:

• úti = outside, being outside
• út = out, movement to the outside

Here í means in, and it is used with months:

• í maí = in May
• í júní = in June

That is very normal Icelandic usage.

More generally, í can mean in / into, depending on context and case, but with months you can simply think of it as in.

Líka means also, and its position is fairly flexible, but it often appears after the finite verb in natural Icelandic word order.

So:

• heldur er líka auðveldara ... = but it is also easier ...

The placement helps the sentence sound natural and keeps the V2 structure intact.

Again, this is because of verb-second word order.

The clause starts with Í júní, so the finite verb comes next:

• Í júní borðum við stundum kvöldmat ...

If the subject came first, it would be:

• Við borðum stundum kvöldmat á svölunum í júní.

Both are grammatical, but when Í júní is moved to the front for emphasis or topic, the verb must come second.

Stundum means sometimes.

In this sentence:

• borðum við stundum kvöldmat = we sometimes eat dinner

It often appears around the middle of the clause, commonly after the subject or after the finite verb plus subject, depending on word order.

Examples:

• Við borðum stundum kvöldmat á svölunum.
• Í júní borðum við stundum kvöldmat á svölunum.

Because borða (to eat) takes a direct object, and here that object is in the accusative case.

The noun is:

• nominative: kvöldmatur = dinner
• accusative: kvöldmat

So:

• Við borðum kvöldmat = We eat dinner

This is a very common pattern in Icelandic: the noun changes form depending on its grammatical role.

Á svölunum means on the balcony / on the balconies, but in this context it is best understood as on the balcony.

The preposition á can take different cases depending on whether it means:

• location = usually dative
• motion onto = usually accusative

Here it describes location, so it uses the dative:

• svalir = balcony / balconies
• svölunum = the balcony/balconies, dative definite form

So:

• á svölunum = on the balcony

This is parallel to patterns like:

• á borðinu = on the table
• á gólfinu = on the floor

The word svalir is one of those Icelandic nouns that is often used in the plural form where English uses a singular idea.

So even though English says the balcony, Icelandic often says svalir and then inflects it in the plural:

• nominative definite: svalirnar
• dative definite: svölunum

This is just something to learn as part of the noun’s normal usage.

Yes. A more straightforward version would be something like:

• Garðurinn er ekki bara grænni í maí, heldur er líka auðveldara að sitja úti, og við borðum stundum kvöldmat á svölunum í júní.

This keeps the same meaning, but the original version sounds more natural and stylistically smooth because it foregrounds the time expressions:

• Í maí ...
• í júní ...

That gives the sentence a nice seasonal flow.