Ég set stól á borðið.

Because stól is the accusative singular form of stóll (chair).

In this sentence, stól is the direct object of the verb set (put / place), and direct objects often appear in the accusative in Icelandic.

So the forms are:

• stóll = nominative singular
• stól = accusative singular

A native English speaker often expects the noun to stay unchanged, but in Icelandic the form changes depending on its role in the sentence.

Because the preposition á can take different cases depending on the meaning.

Here it means movement onto the table, not location on the table. With movement toward a place, á usually takes the accusative.

So:

• á borðið = onto the table
• á borðinu = on the table

This is a very important Icelandic pattern:

• accusative after á = motion toward / onto
• dative after á = location / on

So in Ég set stól á borðið, the chair is being moved onto the table.

Because Icelandic marks definiteness on each noun separately.

In this sentence:

• stól = a chair
• borðið = the table

The -ið on borðið is the definite article attached to the end of the noun. Icelandic usually puts the as a suffix, not as a separate word.

Compare:

• borð = a table / table
• borðið = the table

And similarly:

• stóll = a chair / chair
• stóllinn = the chair
• stólinn = the chair (accusative)

So the sentence specifically means something like I put a chair onto the table, not I put the chair onto the table.

It comes from the verb setja, which means to put, place, set.

In the sentence, set is the present tense, 1st person singular form:

• ég set = I put / I am putting

So the basic pattern is:

• að setja = to put
• ég set = I put
• þú setur = you put
• hann/hún/það setur = he/she/it puts

This is a very common Icelandic verb.

It can mean both, depending on context.

Icelandic often uses the simple present where English might use either:

• I put
• I am putting

So Ég set stól á borðið could mean:

• I put a chair on the table
• I am putting a chair on the table

English distinguishes these more clearly than Icelandic does.

This is the normal subject–verb–object order:

• Ég = subject
• set = verb
• stól = direct object
• á borðið = prepositional phrase

So it works very much like English here:

• I / put / a chair / onto the table

Icelandic word order can change in other sentence types, but this sentence is quite straightforward.

You might sometimes see different word orders in Icelandic for emphasis, style, or information structure, but Ég set stól á borðið is the most neutral and natural basic order.

Putting á borðið before stól would sound marked or unusual in ordinary speech unless there were a special reason to emphasize the destination.

For learners, the safest default is:

subject + verb + object + place/direction phrase

Because Icelandic usually expresses the by attaching it to the end of the noun.

So instead of a separate word like English the table, Icelandic often says:

• borðið = the table

This ending changes with gender, number, and case, so it is not always the same ending, but the general idea is the same: definiteness is often built into the noun.

It is accusative singular definite.

That may look identical to the nominative/accusative indefinite form borð, plus the definite ending -ið, because borð is a neuter noun.

So here:

• á
  • motion = accusative
• borðið = the table in the accusative

With a neuter noun like borð, some forms look the same, which is very common in Icelandic.

That would be:

Ég set stólinn á borðið.

Here:

• stólinn = the chair (accusative singular definite)
• borðið = the table

So the only difference is that now the object is definite.

That would be:

Stóllinn er á borðinu.

Now there is no movement. It is just location. That is why á takes the dative:

• á borðinu = on the table

This is a very useful contrast:

• Ég set stól á borðið. = I put a chair onto the table.
• Stóllinn er á borðinu. = The chair is on the table.

So the case after á helps show the difference between motion and location.

Not dramatically, but the spelling difference matters grammatically.

• stóll is the dictionary form
• stól is the accusative form used here

For a learner, the bigger point is usually not pronunciation but recognizing that Icelandic nouns change form by case.

So when you see stól, think: this is the object form of stóll.

Yes, grammatically it is perfectly natural.

It means that the speaker places a chair onto the table. Whether that is a likely real-world action is a different question, but the Icelandic structure is normal and useful for learning:

• subject
• present-tense verb
• accusative object
• á
  • accusative for motion onto something

So it is a good example sentence because it shows several important grammar points clearly.