Þjálfarinn kom snemma og lagði boltana á gólfið.

Because Icelandic usually puts the definite article on the end of the noun.

• þjálfari = coach
• þjálfarinn = the coach

Here, þjálfarinn is also in the nominative singular because it is the subject of the sentence.

Kom is the 3rd person singular past tense of koma.

• koma = to come
• kemur = comes / is coming
• kom = came

So Þjálfarinn kom means the coach came.

Snemma is an adverb, meaning early. It describes when the coach came.

It does not change for gender, number, or case, so it stays snemma.

Leggja is the infinitive, meaning to lay / to put.
Lagði is the past tense form used here.

• leggja = to put
• leggur = puts / is putting
• lagði = put

So lagði boltana means put the balls.

Because boltana is the accusative plural definite form, and here the balls are the direct object of lagði.

• boltarnir = the balls as a subject
• boltana = the balls as an object

In this sentence, the coach is doing something to the balls, so Icelandic uses the object form: boltana.

The dictionary form is bolti, meaning ball.

Some useful forms are:

• bolti = a ball
• boltinn = the ball
• boltar = balls
• boltana = the balls, accusative plural

So the sentence uses the form needed after the verb lagði.

This is a very common Icelandic pattern with prepositions like á.

• á + accusative often shows movement onto/to something
• á + dative often shows location on something

Here the balls are being moved onto the floor, so Icelandic uses the accusative:

• á gólfið = onto the floor

If the sentence described where the balls already were, you would expect the dative:

• Boltarnir voru á gólfinu = The balls were on the floor

Because the subject is the same in both parts of the sentence.

• Þjálfarinn kom snemma
• og lagði boltana á gólfið

The understood subject of lagði is still þjálfarinn. This works much like English: The coach came early and put the balls on the floor.

Yes, but it is also very normal Icelandic word order.

The sentence follows the usual main-clause pattern:

• Þjálfarinn = subject
• kom = verb
• snemma = adverb
• og lagði boltana á gólfið = coordinated second verb phrase

Icelandic is a V2 language, which means the finite verb often comes in the second position in a main clause. In this sentence, the subject comes first, so the verb naturally follows it.

For example, if you moved snemma to the front, the verb would still come next:

• Snemma kom þjálfarinn og lagði boltana á gólfið.

A few sounds here are especially important for English speakers:

• Þ / þ in Þjálfarinn is like th in thin
• ð in lagði and gólfið is like th in this (though in real speech it can be weaker)
• á is pronounced like ow in now, but longer
• ó is roughly like o in go, but longer

Also, ll in Icelandic can sound different from English l, depending on the word and dialect, so it is worth listening to native audio for words like gólfið.