Síðan nýja sorptunnan kom er auðveldara að flokka plast og pappír.

Here síðan means since.

It introduces a clause that gives the starting point for the situation in the main clause:

Síðan nýja sorptunnan kom = Since the new trash bin arrived

A common source of confusion is that síðan can also mean then or after that in other contexts, so you have to tell from the sentence how it is being used.

Because the noun is definite: it means the new trash bin, not a new trash bin.

In Icelandic, when a noun is definite, the adjective usually takes the weak form:

• ný sorptunna = a new trash bin
• nýja sorptunnan = the new trash bin

So nýja is the correct adjective form here because it modifies a definite noun.

Because sorptunnan is the subject of kom and is therefore in the nominative case.

• sorptunnan = nominative definite singular
• sorptunnuna = accusative definite singular

Since the sentence says the new trash bin arrived, the bin is the thing doing the arriving, so it must be nominative.

Because the sentence talks about:

• a past event: the new trash bin arrived
• a present result: it is easier now to sort plastic and paper

So the tense choice is very natural:

• kom = came / arrived
• er auðveldara = is easier

This is similar to English: Since the new trash bin arrived, it is easier...

Auðveldara is the comparative form of auðveldur = easy.

So:

• auðvelt = easy
• auðveldara = easier

The sentence is saying that sorting plastic and paper is easier than before. Icelandic often leaves that comparison unstated when it is obvious from context.

Because Icelandic often uses the neuter singular form of an adjective in impersonal expressions like:

• Það er auðvelt að... = It is easy to...
• Það er auðveldara að... = It is easier to...

There is no specific masculine or feminine noun that auðveldara is agreeing with here. It works like an impersonal it is easier structure, so the neuter singular form is used.

Icelandic often does not need a dummy subject like English it.

English says:

It is easier to sort plastic and paper.

Icelandic can simply say:

Er auðveldara að flokka plast og pappír.

The infinitive phrase að flokka plast og pappír supplies the real content, so Icelandic does not have to insert það in the same way English does.

Að flokka is the infinitive, meaning to sort.

After adjectives like auðvelt / erfitt / gott / slæmt, Icelandic very often uses að + infinitive:

• auðvelt að lesa = easy to read
• erfitt að skilja = hard to understand
• auðveldara að flokka = easier to sort

So að flokka plast og pappír tells you what is easier.

Because they are being used in a general sense, as categories of material:

• plast = plastic
• pappír = paper

This is like English when talking about materials or recycling categories in general.

If Icelandic used the definite forms, it would sound more like specific plastic or specific paper already known from the context.

Also, after flokka, they function as direct objects, but for these nouns the accusative form looks the same as the basic dictionary form.

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

The opening clause Síðan nýja sorptunnan kom takes the first slot. After that, the finite verb of the main clause comes next:

• first slot: Síðan nýja sorptunnan kom
• second position verb: er
• rest of clause: auðveldara að flokka plast og pappír

So the order is completely normal for Icelandic.

Yes. It is a compound noun:

• sorp = trash, garbage
• tunna = barrel, bin

So sorptunna means trash bin / garbage bin.

Compound nouns are extremely common in Icelandic, and recognizing their parts can make vocabulary much easier to learn.

Sometimes the two can be similar, but they are not exactly the same.

• síðan here means since, with the idea that the result continues up to now
• eftir að usually means after, focusing more on sequence in time

So:

• Síðan nýja sorptunnan kom... = Since the new trash bin arrived...
• Eftir að nýja sorptunnan kom... = After the new trash bin arrived...

In this sentence, síðan is better because the main point is the ongoing result: it has been easier from that time onward.