„Stór“ og „lítill“ eru andheiti.

Because the subject is two things together: stór and lítill.

• er = is (singular)
• eru = are (plural)

So Icelandic treats stór og lítill as a plural subject, just like English does in big and small are opposites.

This is the present tense of að vera (to be):

• ég er = I am
• þú ert = you are
• hann/hún/það er = he/she/it is
• við/vér erum = we are
• þið eruð = you are
• þeir/þær/þau eru = they are

So here, eru simply means are.

They are being given in their basic citation form, the form you would usually learn first for an adjective: masculine singular nominative.

So:

• stór = masculine singular nominative
• lítill = masculine singular nominative

This is the form you would use with a masculine singular noun, for example:

• stór hundur = a big dog
• lítill hundur = a small dog

When Icelandic talks about words themselves, it often uses this basic dictionary-style form.

Yes. Icelandic adjectives change for gender, number, and case.

For example, stór and lítill can appear in different forms depending on the noun:

• stór hundur = big dog
• stór bók would be wrong, because bók is feminine

Instead you get:

• stór bók? No — the correct feminine form is stór in some patterns, but adjectives can vary depending on strong/weak declension, so learners need to learn the full system carefully.
• stórt hús = big house

For lítill:

• lítill hundur = small dog
• lítil bók = small book
• lítið hús = small house

So the sentence uses just one form of each adjective, not all possible forms.

Because Stór is the first word of the sentence.

In Icelandic, as in English, the first word of a sentence is capitalized. The adjective itself is not normally capitalized when used in the middle of a sentence.

So if the same word appeared later, it would usually be:

• stór

not

• Stór

Because the sentence is talking about the words themselves, not using them to describe something.

In English, compare:

• Big and small are antonyms.

Here, big and small are being mentioned as vocabulary items.

Icelandic often marks this with quotation marks, and in your sentence that is exactly what is happening. The words are being presented as words, not as ordinary adjectives modifying nouns.

Andheiti is a noun meaning antonym.

A useful thing to know is that this word is neuter, and its form is often the same in singular and plural. So:

• andheiti = antonym
• andheiti = antonyms

In your sentence, the meaning is clearly plural because there are two words being compared and the verb is plural:

• Stór og lítill eru andheiti.
• Big and small are antonyms.

This same pattern happens with some other Icelandic neuter nouns too.

Because Icelandic does not have an indefinite article like English a/an.

So where English says:

• They are antonyms
• It is an antonym

Icelandic can simply use the noun without an article:

• Þau eru andheiti
• Það er andheiti

Icelandic does have a definite article (the), but it is not used here because the meaning is general: the sentence is classifying the two words as antonyms.

Og means and.

It is one of the most common Icelandic conjunctions.

So:

• stór og lítill = big and small

Very straightforward here.

Yes, at least from a learner’s point of view, lítill is one of the adjectives that feels less regular than something like stór.

Its forms include:

• lítill = masculine
• lítil = feminine
• lítið = neuter

So you should learn it as a full pattern rather than assuming every form will be perfectly predictable.

That said, in this sentence you only need to recognize that lítill is the basic masculine singular nominative form.

A rough guide:

• stór ≈ store but with a purer long o
• lítill ≈ LEE-titl or LEE-till
• eru ≈ EH-ru
• andheiti ≈ ANT-hay-ti or ANT-hay-tih

A few important notes:

• ó is a distinct vowel in Icelandic, not just plain o
• í is like ee
• Icelandic r is usually trilled or tapped more than in most English accents

Pronunciation varies a bit by speaker, so it is best to listen to native audio if possible.

Sometimes, yes, but marking them off makes it much clearer that the speaker is referring to the words themselves.

Without special marking, a learner might briefly try to read stór og lítill as ordinary adjectives describing omitted nouns. The marking tells you immediately: these are lexical items being discussed.

So the marking is helpful and natural in a sentence about vocabulary.