Ég legg lyklana á borðið þegar ég kem heim.

Legg is the normal 1st person singular present tense form of the verb að leggja: ég legg = I lay / I put.

The form leggi does exist, but it is used in other contexts, especially the subjunctive. In a straightforward statement like this one, Icelandic uses the indicative form:

• ég legg = I put
• þú leggur = you put
• hann/hún leggur = he/she puts

So in this sentence, Ég legg lyklana... simply means I put the keys...

Lyklana is the definite accusative plural form of lykill = key.

Here is the logic:

• dictionary form: lykill = a key
• plural: lyklar = keys
• definite plural: lyklarnir = the keys
• accusative definite plural: lyklana = the keys, as a direct object

In the sentence, lyklana is the thing being put somewhere, so it is the direct object of legg. That is why it appears in the accusative.

So:

• Ég legg lyklana... = I put the keys...

This is a very common Icelandic case question.

The preposition á can take different cases depending on meaning:

• accusative when there is movement toward / onto something
• dative when something is located on something

In this sentence, the keys are being moved onto the table, so Icelandic uses the accusative:

• á borðið = onto the table

If you were just describing location, you would use the dative:

• lyklarnir eru á borðinu = the keys are on the table

So the contrast is:

• Ég legg lyklana á borðið = I put the keys onto the table
• Lyklarnir eru á borðinu = the keys are on the table

Borðið is the accusative singular definite form of borð = table.

Because á expresses movement here, it takes the accusative, so borð becomes borðið.

Also, the ending -ið here includes the definite article, so borðið means the table, not just table.

So:

• borð = table
• borðið = the table

And in this sentence:

• á borðið = onto the table

Þegar means when in this sentence.

It introduces a time clause:

• þegar ég kem heim = when I come home

So the whole sentence is built like this:

• Ég legg lyklana á borðið = I put the keys on the table
• þegar ég kem heim = when I come home

Together: I put the keys on the table when I come home.

Kem is the 1st person singular present tense of að koma = to come.

So:

• ég kem = I come
• þú kemur = you come
• hann/hún kemur = he/she comes

In the clause þegar ég kem heim, it literally means when I come home.

English learners sometimes expect something like koma to stay unchanged, but Icelandic verbs change form depending on the subject, so ég kem is the correct finite verb form here.

Because heim already carries the idea of motion homeward.

In Icelandic, heim is a very common directional adverb meaning home in the sense of going/coming home. So:

• ég kem heim = I come home
• ég fer heim = I go home

If you are talking about being at home rather than moving there, you usually use:

• heima = at home

So compare:

• Ég kem heim. = I come home.
• Ég er heima. = I am at home.

That difference is very important in Icelandic.

In normal Icelandic, the subject is usually stated, so ég is repeated because þegar ég kem heim is a separate clause with its own subject and verb.

So Icelandic does not normally omit the subject the way some languages do. You need:

• þegar ég kem heim = when I come home

not just:

• þegar kem heim ✘

Even though English also repeats I, learners sometimes ask this because the subject is already clear from the first clause. In Icelandic, repeating it is still necessary.

Yes. A very natural alternative is:

• Þegar ég kem heim, legg ég lyklana á borðið.

This means the same thing.

The important thing is that Icelandic usually follows a verb-second pattern in main clauses. That means if you put the time clause first, the verb of the main clause comes before the subject:

• Þegar ég kem heim, legg ég...

not:

• Þegar ég kem heim, ég legg... ✘

So both are correct:

• Ég legg lyklana á borðið þegar ég kem heim.
• Þegar ég kem heim, legg ég lyklana á borðið.

The second one may sound a little more natural if you want to emphasize the time.

Most naturally, it sounds habitual or like a general routine:

• I put the keys on the table when I come home.

That is, this is what I usually do.

Icelandic present tense often works like English present tense for habits and general truths. Depending on context, it can also refer to the future, but without extra context this sentence is best understood as a repeated action or usual behavior.

Yes, very often.

Að leggja commonly means to lay, to place, or to put down, especially when placing something somewhere in a fairly neutral way.

For example:

• leggja bók á borðið = put a book on the table
• leggja lykla á borðið = put keys on the table

English often uses put very broadly, while Icelandic may choose among several verbs depending on the kind of action. But leggja is a very normal and useful choice here.

You would switch from an action of movement to a state of location:

• Lyklarnir eru á borðinu.

Why the changes?

• Lyklarnir = the keys, as the subject
• eru = are
• á borðinu = on the table, with dative because this is location, not movement

So this pair is very useful:

• Ég legg lyklana á borðið. = I put the keys onto the table.
• Lyklarnir eru á borðinu. = The keys are on the table.