Ég set plastið í sorptunnuna og geymi gamla kassann í geymsluhólfinu.

Because í can take different cases depending on meaning:

• accusative for movement into something
• dative for location in something

So here:

• í sorptunnuna = into the trash bin → movement, so accusative
• í geymsluhólfinu = in the storage compartment → location, so dative

This is a very common Icelandic pattern with prepositions like í.

Those endings show a combination of:

• gender
• case
• number
• and the definite article (the)

In Icelandic, the is usually attached to the end of the noun instead of appearing as a separate word.

Here is the breakdown:

• plastið = plast
  • definite ending → the plastic
• sorptunnuna = sorptunna in accusative singular definite
• kassann = kassi in accusative singular definite
• geymsluhólfinu = geymsluhólf in dative singular definite

So these are not random spelling changes: they are grammatical endings.

Because the adjective has to match the noun, and here the noun is:

• masculine
• singular
• accusative
• definite

When an adjective comes before a definite noun in Icelandic, it usually takes the weak form.

So:

• gamall kassi = an old box

  • indefinite
  • strong adjective form

• gamla kassann = the old box

  • definite
  • weak adjective form

That is why you see gamla, not gamall.

Because both verbs have the same subject: I.

So Icelandic, like English, can omit the repeated subject in a coordinated sentence:

• Ég set plastið í sorptunnuna og geymi gamla kassann í geymsluhólfinu.

This works just like English:

• I put the plastic in the bin and store the old box in the compartment.

You could repeat ég, but it would usually sound more emphatic or stylistically heavier:

• Ég set plastið í sorptunnuna og ég geymi gamla kassann í geymsluhólfinu.

The version without the second ég is the more natural neutral one.

Because they belong to different verb classes and therefore conjugate differently.

Their dictionary forms are:

• setja = to put
• geyma = to store / keep

Present tense, 1st person singular:

• ég set
• ég geymi

Compare some forms:

• setja → ég set, þú setur, hann/hún/það setur
• geyma → ég geymi, þú geymir, hann/hún/það geymir

So the difference is not about meaning here; it is just how those verbs are conjugated.

Here are the likely dictionary forms you would look up:

• ég → pronoun ég
• set → setja
• plastið → plast
• sorptunnuna → sorptunna
• geymi → geyma
• gamla → gamall
• kassann → kassi
• geymsluhólfinu → geymsluhólf

This is useful because Icelandic words often appear in heavily inflected forms, so the form in the sentence is not always the form you would search for in a dictionary.

Because many neuter singular nouns have the same form in the nominative and accusative.

So plastið can serve as either:

• the plastic as a subject
• the plastic as a direct object

In this sentence it is the direct object of set, but its form does not change.

That is normal for many neuter nouns in Icelandic.

Yes, but this sentence uses a very normal, straightforward word order.

The first clause is:

• Ég = subject
• set = verb
• plastið = object
• í sorptunnuna = prepositional phrase

Then:

• og = and
• geymi = second verb
• gamla kassann = object
• í geymsluhólfinu = prepositional phrase

This is a neutral sentence pattern.

Also, Icelandic main clauses usually follow the verb-second tendency. Since the sentence begins with Ég, the verb set comes right after it. If some other element were moved to the front, the verb would still normally stay in second position.

Yes, it matters a lot, because gender affects articles, adjective forms, and case endings.

In this sentence:

• plast is neuter
• sorptunna is feminine
• kassi is masculine
• geymsluhólf is neuter

You can see the effects clearly:

• plastið → neuter definite form
• sorptunnuna → feminine accusative definite form
• gamla kassann → masculine accusative definite noun, with matching adjective
• geymsluhólfinu → neuter dative definite form

So yes, noun gender is one of the key things you need to know when building Icelandic sentences correctly.