Blóðið er rautt.

-ið is the definite article attached to the end of the noun. Icelandic usually puts the onto the noun instead of using a separate word like English.

So:

• blóð = blood
• blóðið = the blood

In this sentence, -ið is the neuter singular definite ending.

That is a normal feature of Icelandic grammar. The definite article is usually a suffix, not a separate word.

So instead of English:

• the blood

Icelandic normally says:

• blóðið

You will see this all the time in Icelandic:

• hús = house
• húsið = the house

Er is the present tense, 3rd person singular form of vera (to be).

Because blóðið is singular, Icelandic uses singular er:

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

So Blóðið er rautt follows the same basic pattern as English X is Y.

Because the adjective has to agree with the noun.

The noun blóð is neuter singular, so the adjective must also be neuter singular:

• rauður = masculine
• rauð = feminine
• rautt = neuter

Since blóðið is neuter, Icelandic uses rautt.

The dictionary form is rauður.

Icelandic adjectives are usually listed in the masculine singular nominative form, even though in real sentences they change form to match the noun.

So:

• dictionary form: rauður
• used here: rautt

That is completely normal.

Here rautt is a predicate adjective. It comes after er and describes the subject:

• Blóðið er rautt. = The blood is red.

If you put the adjective directly before the noun, it becomes an attributive adjective:

• rautt blóð = red blood

So both patterns exist, but they are used differently:

• blóðið er rautt = the blood is red
• rautt blóð = red blood

Good question. Icelandic adjectives behave differently depending on how they are used.

In Blóðið er rautt, the adjective is predicative because it comes after er. Predicative adjectives normally just agree in gender, number, and case, and here the correct form is rautt.

If the adjective were directly attached to a definite noun, you would often get a weak adjective form, for example:

• rauða blóðið = the red blood

So:

• Blóðið er rautt = predicate adjective
• rauða blóðið = attributive adjective with a definite noun

That is why rautt is correct in your sentence.

Yes. That would mean blood is red in a more general sense.

The difference is roughly:

• Blóð er rautt = blood is red / blood in general is red
• Blóðið er rautt = the blood is red / this particular blood is red

In many learning examples, the definite form is used simply to show the grammar clearly, but in real usage the choice depends on context.

You learn noun gender as part of the word. In dictionaries, blóð is marked as neuter.

In this sentence, you can also see clues:

• the article is -ið, a common neuter singular definite ending
• the adjective is rautt, the neuter form of rauður

So the sentence itself shows that blóð is neuter.

A rough learner-friendly guide is:

• blóðið: the ð sounds like th in this
• er: like eh-r with a short vowel
• rautt: the au is roughly like the vowel in boy, and tt in Icelandic is not always a plain English t

A very rough approximation would be something like:

BLOH-thith er röyt

But Icelandic pronunciation has sounds that do not match English exactly, so it is best to listen to native audio if possible.