Þakið lekur þegar það rignir mikið.

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

• þak = roof
• þakið = the roof

Here -ið is the definite ending for a neuter singular noun in this form. So þakið means the roof, not just roof.

It is in the nominative case because it is the subject of the sentence: the thing that is doing the leaking.

In Þakið lekur, the roof is the subject, so nominative is the expected case.

Also, for many neuter nouns, the nominative and accusative singular look the same, so the form þakið does not by itself tell you everything; its role in the sentence does.

að leka is the infinitive, meaning to leak.
lekur is the present tense form used with a third-person singular subject such as þakið.

So:

• að leka = to leak
• þakið lekur = the roof leaks / the roof is leaking

This is the normal way Icelandic works: dictionary forms end in að + infinitive, but in a real sentence you use a conjugated form.

It can mean either, depending on context.

Icelandic present tense often covers both:

• a general or habitual meaning: the roof leaks when it rains a lot
• a current situation: the roof is leaking when it rains a lot

In this sentence, the most natural reading is a general fact or repeated situation.

Because Icelandic, like English, often uses a dummy subject with weather expressions.

So:

• það rignir = it rains

This það does not refer to any actual thing. It works like English it in it is raining.

Here þegar means when and introduces a subordinate clause:

• þegar það rignir mikið = when it rains a lot

So the structure is:

• main clause: Þakið lekur
• subordinate clause: þegar það rignir mikið

In other contexts, þegar can also mean already, but that is not what it means here.

Here mikið is being used adverbially, meaning a lot or heavily.

In Icelandic, adjectives are often used in the neuter singular when they function like adverbs. So:

• mikið here does not describe a noun directly
• it modifies the verb rignir

So það rignir mikið means:

• it rains a lot
• it rains heavily

This is a very common question.

• mikið usually means a lot, much, or heavily
• mjög usually means very

So:

• það rignir mikið = it rains a lot / heavily
• það er mjög kalt = it is very cold

In this sentence, mikið is the natural choice because it tells you the amount or intensity of the raining itself, not just that something is very something.

Yes:

Þegar það rignir mikið lekur þakið.

That means the same thing.

But notice the word order. After the fronted subordinate clause, Icelandic keeps the finite verb early in the main clause:

• Þegar það rignir mikið lekur þakið

Not:

• Þegar það rignir mikið þakið lekur

This is part of the normal Icelandic verb-second pattern in main clauses.

Because in a sentence like this, Icelandic normally does not need a comma before a subordinate clause introduced by þegar.

So:

• Þakið lekur þegar það rignir mikið.

is the normal way to write it.

If the clause comes first, some writers may use a comma after it, especially for clarity, but often Icelandic uses fewer commas than English learners might expect.

A good approximate guide is:

• þ = the th sound in think
• ð = the th sound in this

So þakið is roughly pronounced like THA-kið, with:

• þ as voiceless th
• ð as voiced th

For an English speaker, that is a good starting point, even though the real Icelandic pronunciation is a bit more precise than English spelling can show.

Most naturally, it describes a general or repeated situation:

• whenever it rains a lot, the roof leaks

That is because the present tense plus þegar often expresses something habitual or generally true.

If you wanted to emphasize a single ongoing event, the context would usually make that clear.