Sekúndan líður hratt.

It is the definite article attached to the noun. Icelandic often puts the at the end of the word instead of using a separate word.

• sekúnda = a second
• sekúndan = the second

With many feminine nouns ending in -a, the nominative singular definite form is made by adding -n.

Sekúnda is a feminine noun. That matters because Icelandic nouns change form depending on gender, number, case, and definiteness.

In this sentence, sekúndan is:

• feminine
• singular
• nominative
• definite

Because it is the subject of the sentence.

In Sekúndan líður hratt, the thing doing the action of passing is the second, so Icelandic uses the nominative for it.

Here it means the time unit: a second.

If you mean second as an ordinal number, Icelandic uses a different word, usually annar or a related form depending on gender and case. So this sentence is definitely about time, not about something being in second place.

It comes from the verb líða.

In this sentence, líður is the present tense, third person singular form, matching the singular subject sekúndan.

So:

• líða = the infinitive
• líður = passes / goes by / elapses

It is a verb whose present-tense form is not completely obvious from the infinitive, so líða → líður is worth memorizing.

No. Líða has more than one common use.

Here it means to pass / to go by / to elapse.

But you will also see it in expressions about how someone feels, for example:

• Mér líður vel = I feel well

So the verb can mean different things depending on the context. In this sentence, the time-related meaning is the right one.

Hratt is an adverb, meaning quickly or fast.

It is related to the adjective hraður = fast. Icelandic often uses the neuter singular form of an adjective as an adverb, so:

• hraður = fast (masculine adjective form)
• hröð = fast (feminine adjective form)
• hratt = fast/quickly (neuter form, and also the adverbial form)

Because hratt is modifying the verb líður, not the noun sekúndan.

Adverbs do not agree with nouns in gender, number, or case. So even though sekúndan is feminine singular, hratt stays the same.

If you were describing the noun itself with an adjective, then agreement would matter.

No. This sentence has:

• a subject: sekúndan
• a verb: líður
• an adverb: hratt

There is no direct object here. The verb líða is being used intransitively.

The given order, Sekúndan líður hratt, is the most neutral and natural order for a simple statement.

Icelandic word order is somewhat flexible, but changes usually affect emphasis and must still follow normal Icelandic syntax. For example:

• Hratt líður sekúndan can be possible for emphasis or a more literary style
• Líður sekúndan hratt? can sound like a question

So the original sentence is the best basic pattern to learn first.

You would remove the definite article:

• Sekúnda líður hratt = A second passes quickly

So the contrast is:

• sekúnda = a second
• sekúndan = the second

You can make both the noun and the verb plural:

• Sekúndur líða hratt = Seconds pass quickly
• Sekúndurnar líða hratt = The seconds pass quickly

Notice that the verb also changes from líður to líða with a plural subject.