Regndroparnir söfnuðust í pollinum fyrir framan innganginn.

Regndroparnir means the raindrops, and it breaks down like this:

• regndropi = a raindrop
• regndropar = raindrops
• regndroparnir = the raindrops

The ending -nir is the definite article here, attached to the noun.
This form is nominative plural, because it is the subject of the sentence.

In Icelandic, the is usually added to the end of the noun instead of being a separate word.

In this sentence:

• Regndroparnir = the raindrops
• pollinum = in the puddle
• innganginn = the entrance

So Icelandic often builds definiteness directly into the noun form.

The verb comes from að safnast, which means to gather, collect, accumulate.

That is different from:

• að safna = to collect something
• að safnast = to gather / collect by itself

Here, the raindrops are not actively collecting something else; they are gathering together, so safnast is the natural verb.

The form söfnuðust is the past tense, used with a plural subject: the raindrops gathered.

The -st in Icelandic often marks what is called the middle voice.

That can give meanings like:

• something happens by itself
• something happens to itself
• a verb becomes more intransitive

So:

• að safna = to collect
• að safnast = to gather together, to accumulate

In this sentence, the raindrops are simply ending up together in one place, so the -st form fits very well.

This is part of Icelandic verb inflection. Many verbs change their stem vowel in certain past forms, so the change from a to ö is not random.

For a learner, the safest approach is:

• learn the basic verb: að safnast
• learn the past form as a whole: söfnuðust

So yes, the spelling changes, but that is a normal part of Icelandic verb patterns.

Because í can take two different cases:

• accusative when there is motion into something
• dative when something is in a place

Here the sentence describes location: the raindrops gathered in the puddle.
That means dative is needed.

So:

• pollur = puddle
• pollinum = in the puddle

fyrir framan is a very common Icelandic expression meaning in front of.

It is best learned as a single unit:

• fyrir framan = in front of

So:

• fyrir framan innganginn = in front of the entrance

Even though it has two words, learners usually treat it like a fixed prepositional phrase.

Innganginn is the form of inngangur used after fyrir framan.

So:

• inngangur = entrance
• innganginn = the entrance

The important point is that fyrir framan takes the accusative, so you get innganginn, not the nominative form.

This is something you usually just learn together with the expression:

• fyrir framan + accusative

The sentence has very normal Icelandic main-clause word order:

• Regndroparnir = subject
• söfnuðust = verb
• í pollinum fyrir framan innganginn = place information

Icelandic main clauses usually put the finite verb in second position.
Since the subject comes first here, the verb naturally comes second.

So this order is straightforward and natural: The raindrops + gathered + where.