Spilliefnið er hættulegt.

Because Icelandic often puts the definite article on the end of the noun instead of using a separate word.

• spilliefni = hazardous substance / pollutant / harmful material
• spilliefnið = the hazardous substance / the pollutant

Here, -ið is the definite article attached to a neuter singular noun.

The sentence gives you two clues:

• the definite ending -ið
• the adjective form hættulegt

Both point to neuter singular.

Also, many Icelandic nouns ending in -efni are neuter. So the dictionary form is spilliefni and its gender is neuter.

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

Since spilliefnið is neuter singular nominative, the adjective must also be neuter singular nominative:

• masculine: hættulegur
• feminine: hættuleg
• neuter: hættulegt

So:

• Vökvinn er hættulegur = the liquid is dangerous
• Sýran er hættuleg = the acid is dangerous
• Spilliefnið er hættulegt = the hazardous substance is dangerous

Two different things are going on here:

1. hættulega is usually an adverb, meaning dangerously, not the adjective dangerous.
2. After vera (to be), Icelandic normally uses the strong adjective form in predicate position, even if the noun is definite.

So Icelandic says:

• Húsið er stórt = the house is big
• Spilliefnið er hættulegt = the hazardous substance is dangerous

Even though the noun is definite, the predicate adjective is still stórt / hættulegt, not a weak definite form.

The dictionary form is hættulegur.

Icelandic adjectives are usually listed in dictionaries in the masculine nominative singular form. From that, you get the other forms:

• hættulegur = masculine
• hættuleg = feminine
• hættulegt = neuter

So if you look up hættulegt, you should search for hættulegur.

It is in the nominative case, because it is the subject of the sentence.

The structure is:

• Spilliefnið = subject
• er = verb
• hættulegt = predicate adjective

After er, the subject stays nominative, and the predicate adjective agrees with it, so hættulegt is nominative too.

Er is the 3rd person singular present tense of 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, er simply means is.

It is part of the predicate, not part of the noun phrase itself.

That is an important distinction:

• Spilliefnið er hættulegt = The hazardous substance is dangerous
• hættulegt spilliefni = dangerous hazardous material / a dangerous harmful substance

In your sentence, the structure is X is dangerous, so hættulegt is a predicate adjective.

Yes, in this sentence it is.

The neutral word order is:

• subject + verb + complement
• Spilliefnið + er + hættulegt

So it matches English The hazardous substance is dangerous very closely.

Icelandic can change word order for emphasis or in certain sentence types, but this version is the normal, straightforward one.

Yes, but the meaning changes.

• Spilliefnið er hættulegt = The hazardous substance is dangerous
  A specific substance is being talked about.

• Spilliefni er hættulegt = Hazardous material is dangerous or A hazardous substance is dangerous
  This sounds more general or indefinite, depending on context.

So the -ið makes the noun definite and more specific.

A few useful pronunciation points for an English speaker:

• Stress usually falls on the first syllable of each word.
• In spilliefnið, ll often sounds somewhat like tl to English ears.
• In spilliefnið, ð is like the th in this.
• In hættulegt, the tt may sound a bit breathy, almost like ht to English ears.

You do not need to get every detail perfect immediately. The most useful things to aim for are:

• strong first-syllable stress
• correct adjective ending -t
• a clear ð sound in -ið