Þegar tjaldið er tilbúið, legg ég svefnpokann minn inn og set bókina við hliðina á honum.

Þegar means when in this sentence. It introduces a time clause:

Þegar tjaldið er tilbúið = When the tent is ready

So the first part of the sentence sets the time for the actions in the main clause.

This is because Icelandic follows a verb-second pattern in main clauses.

When the sentence starts with something other than the subject—here, the whole clause Þegar tjaldið er tilbúið—the finite verb of the main clause comes next:

• Þegar tjaldið er tilbúið, legg ég ...

If the sentence started with the subject, you would get:

• Ég legg svefnpokann minn inn þegar tjaldið er tilbúið.

So legg ég is normal Icelandic word order after an introductory clause.

The -ið is the suffixed definite article, so tjaldið means the tent.

The dictionary form is:

• tjald = tent

With the definite article:

• tjaldið = the tent

Icelandic usually attaches the to the end of the noun instead of using a separate word like English does.

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

The noun tjaldið is neuter singular, so the adjective must also be in the neuter singular form:

• tilbúinn = masculine
• tilbúin = feminine
• tilbúið = neuter

So:

• tjaldið er tilbúið = the tent is ready

They are both direct objects, so they appear in the accusative singular.

Dictionary forms:

• svefnpoki = sleeping bag
• bók = book

In this sentence:

• legg ég svefnpokann minn inn
• set bókina við hliðina á honum

Because leggja and setja take direct objects, the noun forms change:

• svefnpoki → svefnpokann = the sleeping bag
• bók → bókina = the book

So this is both case marking and the suffixed definite article.

In everyday Icelandic, possession is very often expressed as:

definite noun + possessive

So:

• svefnpokann minn = my sleeping bag

This is the most natural neutral wording here.

The version with the possessive first, such as minn svefnpoki, is possible, but it often feels more emphatic, contrastive, or literary.

Also notice that the noun is in the accusative here, so you get:

• nominative: svefnpokinn minn
• accusative: svefnpokann minn

Inn means in / inside / inward, with a sense of movement into something.

So:

• legg svefnpokann minn inn = I put my sleeping bag in

It implies motion inward, not just location.

A useful contrast is:

• inn = motion inward
• inni = being inside

For example:

• Ég fer inn. = I go in.
• Ég er inni. = I am inside.

It could, but it does not have to.

Because the tent has just been mentioned, Icelandic can leave the destination understood:

• legg svefnpokann minn inn = put my sleeping bag inside

If you want to be more explicit, you could say:

• legg svefnpokann minn inn í tjaldið

That means more literally put my sleeping bag into the tent.

So the shorter version sounds natural because the context already makes the destination clear.

Both verbs can mean put, but they are not always used in exactly the same way.

• leggja often suggests laying something down
• setja is a very common general verb for put / set / place

A sleeping bag is something you naturally lay down, so leggja fits well.

A book can simply be set/placed somewhere, so setja sounds natural there.

This is not a rigid rule in every situation, but the verb choice here is very idiomatic.

Við hliðina á means next to or beside.

So:

• við hliðina á honum = next to it / beside it

It is best learned as a fixed expression. Literally, it is something like by the side of, but in normal English the natural translation is next to.

Honum is the dative singular form of hann.

In this sentence, it refers to svefnpokann (the sleeping bag), not tjaldið (the tent).

Why?

• svefnpoki is a masculine noun
• tjald is a neuter noun

Since honum is masculine, it matches svefnpokann.

This is an important Icelandic point: pronouns agree with grammatical gender, not whether something is actually male or female. So even an inanimate object can be referred to with hann / honum if the noun is masculine.

Also, the phrase við hliðina á requires this form, so you get á honum here.

If the sentence were referring to the tent, you would expect því, not honum.

These are two Icelandic letters that often interest English speakers:

• þ is pronounced like the th in thing
• ð is pronounced roughly like the th in this

Examples from the sentence:

• Þegar starts with þ
• tjaldið, tilbúið, við, and hliðina contain ð

One small warning: ð is often softer than English speakers expect, and in some positions it can sound very weak. But as a first approximation, thing-th for þ and this-th for ð is a good start.