Fyrsti regndropinn féll á bókina mína þegar við gengum út.

Because fyrsti has to agree with regndropinn in gender, number, case, and definiteness.

Here, regndropinn is:

• masculine
• singular
• nominative
• definite (the raindrop)

When an adjective comes before a definite noun in Icelandic, it usually takes the weak form. So:

• fyrstur = strong form, masculine nominative singular
• fyrsti = weak form, masculine nominative singular

That is why you get:

• fyrsti regndropinn = the first raindrop

not fyrstur regndropinn.

The ending -inn is the suffixed definite article in Icelandic. Instead of usually having a separate word for the, Icelandic often adds it to the end of the noun.

So:

• regndropi = raindrop
• regndropinn = the raindrop

This is very common in Icelandic:

• bók = book
• bókin = the book

So regndropinn literally means raindrop-the, but in normal English we translate it as the raindrop.

Because it is the subject of the sentence — it is the thing doing the action of falling.

In the sentence:

• Fyrsti regndropinn féll ...
• The first raindrop fell ...

the raindrop is what fell, so Icelandic uses the nominative for that noun.

Because á can take different cases depending on whether it expresses motion toward a surface or location on a surface.

Here is the key contrast:

• á + accusative = movement onto something
• á + dative = being on something

In this sentence, the raindrop fell onto the book, so there is movement toward the book:

• á bókina = onto the book

If you were talking about something already resting on the book, you would use the dative:

• á bókinni = on the book

This accusative/dative contrast with prepositions is a very common Icelandic pattern.

All three are possible in Icelandic, but they are used a little differently.

In this sentence, bókina mína is the natural form because:

• bókina is the book in the accusative singular
• mína agrees with it in gender, number, and case
• the possessive comes after the noun

So:

• bókina mína = my book / the book of mine

Compare:

• bókin mín = my book (nominative, not accusative)
• mín bók = my book, often less definite in feel or more emphatic/literary depending on context

Since this sentence needs the accusative after á (because of motion), both words have to match that role:

• bókina mína

Because mína must agree with bókina.

Bókina is:

• feminine
• singular
• accusative

So the possessive pronoun minn has to appear in its matching form:

• minn = masculine nominative singular
• mín = feminine nominative singular
• mína = feminine accusative singular

That is why the sentence has:

• á bókina mína

not just mín.

Féll is the past tense of falla (to fall).

Icelandic has many strong verbs, which change their vowel in the past tense. So the forms are not always predictable from the infinitive.

For example:

• falla = to fall
• fellur = falls
• féll = fell
• fallið = fallen

So féll may look quite different from falla, but this kind of vowel change is normal in Icelandic strong verbs.

It is a verb plus a particle:

• ganga = to walk, go
• út = out

Together, ganga út means to go out / to walk out.

In the past tense, first person plural:

• við gengum út = we went out

So it works a lot like English go out.

Also, Icelandic often keeps these particles separate, especially in ordinary sentence structure, so gengum út is completely normal.

Because þegar introduces a subordinate clause: when we went out.

In that clause:

• þegar = when
• við = we
• gengum = went
• út = out

So the word order is straightforward:

• conjunction + subject + verb + particle

This is different from the main clause, where Icelandic often follows verb-second patterns. In subordinate clauses, the order is often more like what you see here:

• þegar við gengum út

That is a very common and natural structure.

Usually, yes — in a sentence like this, þegar means when.

Here it connects two past events:

• the raindrop fell
• we went out

So the meaning is:

• when we went out

Depending on context, þegar can sometimes feel like as, once, or at the time when, but when is the basic meaning and the best one to learn first.

Yes. Icelandic forms many words by combining smaller words.

Here:

• regn = rain
• dropi = drop

So:

• regndropi = raindrop
• regndropinn = the raindrop

Compound nouns are extremely common in Icelandic, and learning to spot them can really help with vocabulary.

Because the whole scene is being described as something that happened in the past.

• féll = fell
• gengum út = we went out

The sentence presents two past actions happening at roughly the same time:

• the first raindrop fell
• at the moment we went out

So using the past tense in both clauses is exactly what you would expect.

Then Icelandic would probably use the dative after á, because it would describe location, not movement.

For example:

• Fyrsti regndropinn var á bókinni minni.
• The first raindrop was on my book.

Notice the difference:

• á bókina mína = onto my book
• á bókinni minni = on my book

That contrast is a very useful one to remember.