Hún spyrnti boltanum fast og skoraði fyrsta markið.

Boltanum is dative singular definite of bolti (ball).

That happens because the verb spyrna in this use takes a dative object. So Icelandic does not say the equivalent of she kicked the ball with an accusative here; it uses dative:

• bolti = a ball
• boltanum = the ball (dative singular)

A very useful thing to learn is the verb together with the case it wants:

• spyrna e-u = to kick something
  (e-u is a common dictionary shorthand for a dative thing)

So in this sentence, hún spyrnti boltanum is grammatically natural because boltanum is the object required by spyrna.

Because the second verb, skora, takes a different case from spyrna.

• spyrna e-u → dative
• skora e-ð → accusative

So:

• boltanum is dative because of spyrnti
• markið is accusative because of skoraði

This is very common in Icelandic: different verbs require different cases for their objects, and you often just have to learn them with the verb.

Spyrnti is the past tense form of spyrna.

In this sentence it means that the action is completed in the past: she kicked.

So the basic breakdown is:

• spyrna = to kick, thrust with the foot
• spyrnti = kicked

Here it agrees with hún (she) as a 3rd person singular past form.

Here fast is an adverb, modifying the verb spyrnti. It tells you how she kicked.

So it works like English hard in kicked the ball hard.

This is a common pattern in Icelandic: a form that looks like the neuter adjective form is often also used adverbially.

So:

• adjective use: describes a noun
• adverb use: describes a verb

In this sentence, fast does not describe the ball; it describes the kicking.

Because the subject is still the same.

The sentence has two coordinated verbs:

• spyrnti
• skoraði

They are linked by og (and), and both actions are done by the same person, hún. Icelandic, like English, often leaves out the repeated subject when it is obvious:

• Hún spyrnti boltanum fast og skoraði fyrsta markið.

This is exactly like English She kicked the ball hard and scored the first goal, not She kicked the ball hard and she scored the first goal.

Because markið is a neuter singular noun, and fyrsta is the form that matches it here.

The word fyrstur (first) changes form for gender, number, and case. In this sentence it is used before a noun as an ordinal, and the correct matching form is fyrsta.

Compare:

• fyrsti = masculine singular in certain forms
• fyrsta = neuter singular here

Since mark is a neuter noun, you get:

• fyrsta markið = the first goal

So fyrsti markið would be wrong because fyrsti does not match the gender/form of markið.

Because Icelandic usually expresses the definite article as a suffix attached to the noun, not as a separate word like English the.

So:

• bolti = ball
• boltanum = the ball (dative singular)

and

• mark = goal
• markið = the goal (nominative/accusative singular neuter)

The exact ending changes depending on gender, number, and case.

That means forms like -num and -ið are not random; they are part of the noun’s full grammatical form.

Here it is accusative, because it is the object of skoraði.

But you are right that the form looks the same. For many neuter singular nouns in Icelandic, nominative and accusative are identical.

So for mark:

• nominative singular definite: markið
• accusative singular definite: markið

You tell which case it is from its role in the sentence, not from the ending alone. Here it is the thing being scored, so it is the object, therefore accusative.

The sentence follows a very natural Icelandic main-clause order:

• Hún = subject
• spyrnti = verb
• boltanum = object
• fast = adverb
• og skoraði fyrsta markið = coordinated second verb phrase

So the structure is roughly:

Subject + Verb + Object + Adverb + and + Verb + Object

That is quite close to English here, which is why this sentence feels relatively straightforward.

Sometimes yes, but the construction is not exactly the same.

Spyrna and sparka are related in meaning, but they do not always behave the same way grammatically.

In this sentence, spyrna is used with a dative object:

• spyrnti boltanum

With sparka, Icelandic often uses a different construction, commonly with í:

• sparkaði í boltann

So even if the overall meaning is similar, you cannot always swap the verbs without changing the grammar. This is another good example of why it helps to learn Icelandic verbs together with the case or preposition pattern they require.