Ég þarf nýtt sjónpróf áður en ég kaupi ný gleraugu.

Because sjónpróf is a neuter singular noun, and the adjective has to agree with it.

Here:

• sjónpróf = a neuter noun
• it is the object of þarf, so it is in the accusative singular
• the strong adjective form for neuter singular accusative is nýtt

So:

• nýr = masculine singular nominative
• ný = several other forms, but not the one needed here
• nýtt = the correct form for sjónpróf in this sentence

It is in the accusative.

The verb þurfa normally takes a direct object in the accusative, and here nýtt sjónpróf is what the speaker needs.

So:

• Ég þarf = I need
• nýtt sjónpróf = the thing needed, so it is the object

A useful thing to notice: the noun sjónpróf looks the same in the nominative and accusative singular, so the case is shown more clearly by the adjective nýtt and by the verb’s pattern.

Icelandic normally has no indefinite article. English uses a/an, but Icelandic usually does not.

So:

• nýtt sjónpróf = a new eye test
• ný gleraugu = new glasses

If Icelandic wants to say the, it usually adds the definite article to the noun itself rather than using a separate word.

It is a compound noun:

• sjón = sight / vision
• próf = test / exam

So sjónpróf is literally something like vision test or eye test.

This kind of compound word is very common in Icelandic.

Because kaupi is the conjugated verb form, while kaupa is the dictionary form (the infinitive).

• kaupa = to buy
• ég kaupi = I buy

So after the subject ég, you need the finite verb form:

• ég kaupi not
• ég kaupa

This is similar to the difference between English to buy and I buy, except Icelandic changes the verb form more consistently.

Yes. It is a present-tense form, but Icelandic often uses the present to talk about a future action when the time is clear from context.

Here the phrase áður en already makes the time relationship clear:

• first: I need a new eye test
• later: I buy new glasses

So ég kaupi is grammatically present, but in meaning it works like English before I buy or even before I’m going to buy.

Áður en means before when it introduces a whole clause.

In this sentence:

• áður en ég kaupi ný gleraugu = before I buy new glasses

Compare:

• áður by itself = earlier / before
• áður en = before + clause

So the en is important here because it connects the second clause.

Because áður en normally introduces a clause with its own subject and verb.

So Icelandic says:

• áður en ég kaupi ný gleraugu literally:
• before I buy new glasses

The clause has:

• subject: ég
• verb: kaupi

This is very natural Icelandic structure.

Because after a conjunction like áður en, Icelandic normally has regular subordinate-clause order:

• conjunction: áður en
• subject: ég
• verb: kaupi

So:

• áður en ég kaupi ... = correct

For learners, the main thing to remember is that once áður en starts the clause, the subject usually comes before the verb.

Because gleraugu is one of those nouns that is normally used in the plural, just like English glasses.

Even if you mean one pair, Icelandic still says:

• gleraugu

So:

• ný gleraugu = new glasses / a new pair of glasses

This is a very useful vocabulary item to memorize as a plural word.

Because the adjective has to agree with gleraugu.

Here:

• gleraugu is plural
• it is the object of kaupi, so it is accusative
• the adjective takes the matching plural form ný

So:

• ný gleraugu = new glasses

Even though ný may look simpler than nýtt, it is still a specific agreement form.

In this sentence, þarf means need.

• Ég þarf nýtt sjónpróf = I need a new eye test

Depending on context, þurfa can sometimes feel close to have to / must, but here it is most naturally understood as a straightforward need.

So a learner should read this sentence as:

• I need a new eye test before I buy new glasses.

The letter þ is called thorn, and it sounds like the th in think, not the th in this.

So þarf begins with that voiceless th sound.

A rough English-style approximation of þarf would be:

• tharf

This letter is very common in Icelandic, so it is worth getting used to early.