Þegar pelinn er tómur, legg ég hann í vaskinn.

Þegar means when here. It introduces a subordinate time clause:

Þegar pelinn er tómur = When the bottle is empty

That clause sets the time or condition for the main clause:

legg ég hann í vaskinn = I put it in the sink

So the whole sentence has the structure:

When X happens, I do Y.

Not usually. Peli most often means a feeding bottle / baby bottle rather than just any bottle in general.

So if you see pelinn, the image is often more like a baby’s bottle than, say, a soda bottle or wine bottle.

A more general word for bottle is often flaska.

Because pelinn means the bottle, while peli means a bottle or just bottle as the dictionary form.

In Icelandic, the definite article is usually added to the end of the noun rather than written as a separate word like English the.

So:

• peli = bottle
• pelinn = the bottle

In this sentence, the bottle is a specific one, so the definite form pelinn is used.

Because Icelandic adjectives must agree with the noun they describe in gender, number, and usually case.

Here, pelinn is:

• masculine
• singular
• nominative

So the adjective takes the matching form:

• tómur = masculine singular nominative

Compare:

• pelinn er tómur = the bottle is empty
• flaskan er tóm = the bottle is empty, if the noun were feminine
• glasið er tómt = the glass is empty, if the noun were neuter

This is because Icelandic main clauses normally follow a verb-second pattern.

If the sentence started directly with the subject, you would say:

Ég legg hann í vaskinn. = I put it in the sink.

But here the sentence begins with the subordinate clause:

Þegar pelinn er tómur, ...

After that opening element, the finite verb of the main clause comes next:

Þegar pelinn er tómur, legg ég hann í vaskinn.

So legg ég is the normal word order here.

Because Icelandic uses grammatical gender much more than English does.

The noun peli is masculine, so when you refer back to it with a pronoun, you use the masculine form:

• hann = he / it for a masculine noun

So in this sentence, hann means it, referring to pelinn.

This does not mean the bottle is male in any real-world sense. It is just grammatical gender.

Here hann is the direct object of legg, so it is in the accusative.

For this masculine singular pronoun, the accusative form happens to be the same as the nominative form:

• nominative: hann
• accusative: hann

So even though it looks the same, its role in the sentence is different. Here it means I put him/it — so it is the object, not the subject.

Because í can take different cases depending on meaning:

• accusative for movement into
• dative for location in

Here the bottle is being moved into the sink, so Icelandic uses the accusative:

• í vaskinn = into the sink

If you were just saying that it is already there, you would use the dative:

• í vaskinum = in the sink

For example:

• Ég legg hann í vaskinn. = I put it into the sink.
• Hann er í vaskinum. = It is in the sink.

Legg is the 1st person singular present form of the verb leggja.

So:

• leggja = to lay, put, place
• ég legg = I lay / I put / I place

In everyday English, the best translation here is simply I put.

leggja often suggests placing something down somewhere. In many situations Icelandic can also use setja for put/set, but leggja is very natural when placing an object down.

Here is a helpful breakdown:

• pelinn — nominative singular
  It is the subject of er tómur.

• hann — accusative singular
  It is the direct object of legg.

• vaskinn — accusative singular
  It follows í because the meaning is movement into the sink.

So the sentence gives you a nice example of how Icelandic case marking works in a very ordinary everyday sentence.

The comma separates the opening subordinate clause from the main clause:

• Þegar pelinn er tómur, = subordinate clause
• legg ég hann í vaskinn. = main clause

This is normal Icelandic punctuation in a sentence like this. It helps show where the first clause ends and the main statement begins.