Garðurinn er grænni í júní en í apríl.

Because Icelandic usually puts the on the end of the noun instead of using a separate word.

• garður = garden / yard
• garðurinn = the garden / the yard

So -inn is the masculine singular definite article here.

Also, garðurinn is the subject of the sentence, so it stays in the nominative form. That is why you see garðurinn, not garðinn.

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

So:

• garðurinn er... = the garden is...

It is the same verb you see in many basic Icelandic sentences.

Because the sentence is making a comparison: the garden is greener in June than in April.

• grænn = green
• grænni = greener

So the adjective has to be in the comparative form, not the basic form.

Not all Icelandic adjectives form the comparative in exactly the same way. Some use forms like -ari, but others have different comparative patterns.

For grænn (green), the normal comparative is:

• grænn = green
• grænni = greener

So grænni is simply the correct comparative form to learn for this adjective.

Yes. In Icelandic, adjectives usually agree with the noun they describe in gender, number, and often case, even when they come after to be.

Here:

• garðurinn is masculine singular
• so the predicate adjective is grænni

If the subject were neuter singular, you would expect a different form.

Here en means than.

So:

• grænni ... en ... = greener ... than ...

Be careful: en can also mean but in other sentences. After a comparative adjective, though, it usually means than.

Because Icelandic uses í for time expressions like in June and in April.

So:

• í júní = in June
• í apríl = in April

This is very similar to English in June / in April.

Because each month phrase needs its own preposition.

The comparison is really between:

• í júní = in June
• í apríl = in April

So Icelandic normally says:

• í júní en í apríl

not just í júní en apríl.

With time expressions like this, í takes the dative. However, some nouns have the same form in the dative as in the dictionary form, or at least no obvious change in this context.

So even though there is a case relationship here, you do not see a visible ending change in these month names.

That means:

• í júní
• í apríl

look simple, even though case is still involved grammatically.

Because in Icelandic, names of months are normally written with lowercase letters, not capitals.

So Icelandic writes:

• í júní
• í apríl

where English writes:

• in June
• in April

This is completely normal in Icelandic spelling.

This sentence uses very normal Icelandic word order:

• Garðurinn = subject
• er = verb
• grænni = complement
• í júní en í apríl = time comparison

So the structure is basically:

The garden is greener in June than in April.

Icelandic word order can change for emphasis, but this version is the most straightforward and neutral one for a learner to understand.