Dótið er í kassanum.

Because -ið is the definite article for this form of a neuter noun.
The base noun is dót, which often means stuff, things, or sometimes toys depending on context.

So:

• dót = stuff / things
• dótið = the stuff / the things

In Icelandic, the word for the is often attached to the end of the noun instead of being a separate word.

It is grammatically singular, even though in English it may refer to more than one thing.

That is because dót is often used like a collective noun or a mass noun:

• Dótið er... = The stuff is...

So the verb is singular:

• er = is

Even if the meaning feels plural in English, Icelandic treats dótið here as singular.

Er means is. It is the present tense of the verb vera (to be).

A few useful forms are:

• ég er = I am
• þú ert = you are
• hann/hún/það er = he/she/it is

In Dótið er í kassanum, er simply links the subject to its location:

• The stuff is in the box.

Because after the preposition í (in), Icelandic changes the noun depending on meaning and grammar.

Here, kassanum is the dative singular definite form of kassi (box).

So:

• kassi = a box
• kassinn = the box
• kassanum = in the box / to the box depending on context and case use

In this sentence, í shows location, so it takes the dative:

• í kassanum = in the box

Because í is one of the Icelandic prepositions that can take different cases depending on whether you mean:

• location = where something is
• motion toward = where something is going

Here it means location:

• Dótið er í kassanum. = The stuff is in the box.

So Icelandic uses the dative.

If you meant into the box, Icelandic would usually use the accusative instead:

• Dótið fer í kassann. = The stuff goes into the box.

This is a very important pattern in Icelandic.

The difference is:

• í kassanum = in the box → location, so dative
• í kassann = into the box → movement toward, so accusative

Compare:

• Dótið er í kassanum. = The stuff is in the box.
• Ég set dótið í kassann. = I put the stuff into the box.

So the ending helps tell you whether something is already somewhere or moving there.

That is a normal feature of Icelandic. The definite article is usually attached to the noun.

So instead of a separate word like English the, Icelandic often uses noun endings:

• kassi = a box
• kassinn = the box
• kassanum = the box in the dative singular

This happens with many nouns. Learners often need to memorize both the noun and its common case forms.

Here dótið is in the nominative, because it is the subject of the sentence.

The basic structure is:

• Dótið = subject
• er = verb
• í kassanum = prepositional phrase showing location

So the sentence is built very much like English:

• The stuff
  • is
    • in the box

Sometimes, yes, depending on context.

Dót is a broad word. It can mean:

• stuff
• things
• odds and ends
• sometimes toys

So if the conversation is about children's playthings, dótið could naturally be understood as the toys.
But in many contexts it is more general, like the stuff or the things.

A rough pronunciation guide is:

• Dótið ≈ DOH-tith
• er ≈ ehr
• í ≈ ee
• kassanum ≈ KAHS-sa-num

A few helpful notes:

• ó is like o in go, but longer.
• ð is like the th in this, though in some positions it can sound weaker.
• í is a long ee sound.
• ss is pronounced like English s in hiss.

A rough full version could be:

• DOH-tith ehr ee KAHS-sa-num

Yes. Icelandic allows more flexibility than English, but the verb usually still follows the V2 pattern in main clauses, meaning the finite verb tends to come in the second position.

The basic sentence is:

• Dótið er í kassanum.

You could also front the location for emphasis:

• Í kassanum er dótið.

That still means essentially The stuff is in the box, but with more emphasis on in the box.

Yes. Kassi is masculine.

That matters because noun gender affects:

• the form of the definite article
• adjective endings
• case endings

Here are some useful forms:

• kassi = nominative singular
• kassa = accusative singular
• kassa = dative singular without article in some contexts
• kassinn = the box
• kassann = the box (accusative)
• kassanum = the box (dative)

So the ending in í kassanum tells you both case and definiteness.

In principle, you can build them from grammar rules, but in practice it is best to learn both:

• the base noun
• its gender
• its most common case forms

For example, it helps to learn:

• kassi — masculine
• kassinn
• kassann
• kassanum

That way, when you see a sentence like Dótið er í kassanum, you recognize it immediately instead of trying to calculate the form from scratch every time.