Sekkjatrillan er í flutningabílnum.

Because the noun is definite: it means the sack trolley / the hand truck, not just a sack trolley.

In Icelandic, the definite article is usually added to the end of the noun instead of being a separate word like English the.

So:

• sekkjatrilla = a sack trolley / hand truck
• sekkjatrillan = the sack trolley / hand truck

Here, -an is the definite ending for this feminine noun in the nominative singular.

That ending shows two things at once:

1. the noun is definite = the moving truck / transport truck
2. it is in the dative singular

So:

• flutningabíll = a moving truck / transport truck
• flutningabílnum = in the moving truck / transport truck (literally, the moving truck in dative)

The reason it is dative is the preposition í, which often takes the dative when it means in in the sense of location.

Because this sentence describes where something is, not where something is going.

In Icelandic:

• í + dative = in / inside, for location
• í + accusative = into, for motion toward the inside

So:

• Sekkjatrillan er í flutningabílnum.
  = The sack trolley is in the truck.
  → location, so dative

Compare:

• Þeir setja sekkjatrilluna í flutningabílinn.
  = They put the sack trolley into the truck.
  → motion, so accusative

This location-vs-motion difference is very important in Icelandic.

The basic forms are:

• sekkjatrilla = sack trolley / hand truck
• flutningabíll = moving truck / transport truck

These are the forms you would normally look up in a dictionary.

In the sentence, they appear in changed forms because of definiteness and case:

• sekkjatrilla → sekkjatrillan
• flutningabíll → flutningabílnum

Er means is. It is a form of the verb vera, which means to be.

So:

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

In this sentence, sekkjatrillan is singular, so Icelandic uses the singular verb form er.

Because Icelandic usually puts the onto the end of the noun as a suffix.

English:

• the truck
• the trolley

Icelandic:

• bíllinn = the car/truck
• trillan = the trolley

In your sentence, both nouns are definite:

• sekkjatrillan = the sack trolley
• flutningabílnum = the moving truck (in dative form)

So Icelandic often expresses what English does with a separate word by changing the ending of the noun instead.

No, this is a very normal Icelandic sentence pattern:

• Sekkjatrillan = subject
• er = verb
• í flutningabílnum = prepositional phrase showing location

So the structure is basically:

Subject + verb + location

That is very similar to English:

The sack trolley + is + in the moving truck

Icelandic can vary word order more than English in some contexts, but this sentence is straightforward and neutral.

Yes, it matters a lot in Icelandic.

Here:

• sekkjatrilla is feminine
• flutningabíll is masculine

Gender affects things like:

• the form of the definite ending
• adjective endings
• pronouns
• case forms

For example, the definite nominative singular ending on a feminine noun like sekkjatrilla is different from what you would see on many masculine or neuter nouns.

So learning the gender of each noun is important from the start.

Yes. Icelandic uses compound words very often.

flutningabíll is made up of:

• flutninga- = related to transport / moving / hauling
• bíll = car, truck, vehicle

Together, flutningabíll means something like moving truck or transport truck.

Likewise, sekkjatrilla is also a compound noun.

Being able to spot compounds is very useful in Icelandic, because many long words are built from smaller familiar parts.

You would normally remove the definite endings:

Sekkjatrilla er í flutningabíl.

That gives you the indefinite meaning:

• sekkjatrilla = a sack trolley
• flutningabíl = in a moving truck

Notice that flutningabíl is still in the dative singular after í, because the meaning is still location. The only thing that changes is that it is no longer definite.

Then Icelandic would normally use í with the accusative, not the dative.

For example:

Sekkjatrillan fer í flutningabílinn.
= The sack trolley goes into the moving truck.

Compare the two:

• í flutningabílnum = in the truck → location → dative
• í flutningabílinn = into the truck → motion → accusative

This is one of the most important patterns to learn with Icelandic prepositions.