Ég læt símann liggja á náttborðinu í kvöld.

In Icelandic, the present tense is often used for a planned or intended future action when the time is clear from context.

Here, í kvöld (tonight / this evening) makes it clear that the action is future-oriented, so Ég læt... í kvöld can naturally mean I’ll leave... tonight.

This is similar to English sentences like I’m staying home tonight, where a present form refers to the future.

This is a very common Icelandic structure.

láta can mean let, make, have, or cause, depending on context. In this sentence:

• Ég læt
• símann
• liggja

the literal structure is something like I let/cause the phone to lie.

Idiomatic English would usually say I leave the phone lying... or simply I leave the phone...

So the pattern is:

láta + object + infinitive
= to let / make / cause something to do something or be in some state

Here it means that the speaker is intentionally leaving the phone in that position.

This is an important Icelandic distinction:

• liggja = to lie, to be lying
• leggja = to lay, to put

So:

• síminn liggur á borðinu = the phone is lying on the table
• ég legg símann á borðið = I put the phone onto the table

In your sentence, the phone is the thing that is in the lying position, so liggja is the right verb.

Compare:

• Ég læt símann liggja á náttborðinu
  = I leave the phone lying on the bedside table

but:

• Ég legg símann á náttborðið
  = I put the phone onto the bedside table

The first emphasizes the resulting position/state; the second emphasizes the act of placing it there.

símann is the accusative singular definite form of sími (phone).

That happens because it is the direct object of læt in this sentence.

Forms of the noun look like this:

• sími = a phone / the phone (dictionary form, nominative singular)
• síma = a phone (accusative singular, indefinite)
• símann = the phone (accusative singular, definite)

So Ég læt símann liggja... means I leave the phone...

The -nn is the suffixed definite article, which Icelandic usually attaches to the end of the noun instead of using a separate word like the.

Because á changes case depending on whether you mean location or motion.

With á:

• accusative = motion onto something
• dative = location on something

Here the phone is being left on the bedside table as a location, so Icelandic uses the dative:

• á náttborðinu = on the bedside table

Compare:

• Ég legg símann á náttborðið.
  = I put the phone onto the bedside table.
  (motion, so accusative)

• Síminn liggur á náttborðinu.
  = The phone is lying on the bedside table.
  (location, so dative)

It is the dative singular definite form of náttborð, which is a neuter noun meaning bedside table / nightstand.

A simplified set of forms is:

• náttborð = bedside table (nominative/accusative singular, indefinite)
• náttborðið = the bedside table (nominative/accusative singular, definite)
• náttborðinu = the bedside table (dative singular, definite)

It appears in the dative here because of á used for static location.

í kvöld is the normal Icelandic expression for tonight / this evening.

It is one of several common time expressions with í, such as:

• í dag = today
• í morgun = this morning
• í kvöld = tonight

So this is something you should learn as a fixed expression. It is not literally in evening in the way an English speaker might try to analyze it word by word; it is simply the standard Icelandic way to say tonight.

Yes. Icelandic word order is flexible, but it follows the V2 rule in main clauses: the finite verb usually comes in the second position.

So you can say:

• Ég læt símann liggja á náttborðinu í kvöld.

But you can also move the time phrase to the front:

• Í kvöld læt ég símann liggja á náttborðinu.

Notice that when Í kvöld comes first, læt still stays in second position, and ég comes after the verb.

So Í kvöld ég læt... would be wrong.

Yes, but the meaning is slightly less specific.

• Ég læt símann liggja á náttborðinu í kvöld
  emphasizes the phone’s position: it is lying there.

• Ég læt símann vera á náttborðinu í kvöld
  means more generally I’ll leave the phone on the bedside table tonight.

Icelandic often likes to use positional verbs such as:

• liggja = lie
• standa = stand
• sitja = sit

So liggja sounds very natural for an object like a phone resting flat on a surface.