Kaupandinn spyr um verðið.

Because -inn is the definite article attached to the noun. Icelandic usually says the by adding it to the end of the word instead of putting a separate word in front.

So:

• kaupandi = buyer
• kaupandinn = the buyer

This is very common in Icelandic.

Because verðið means the price, while verð just means price.

So:

• verð = price
• verðið = the price

Since the sentence refers to a specific price, Icelandic uses the definite form verðið.

Because the preposition um takes the accusative case, and the accusative singular definite form of verð is verðið.

For this noun:

• nominative/accusative definite singular: verðið
• dative definite singular: verðinu

So after um, you use verðið, not verðinu.

Spyr is the present tense of the verb spyrja, meaning to ask.

In this sentence, spyr is third person singular present, so it means asks:

• ég spyr = I ask
• þú spyrð = you ask
• hann/hún/það spyr = he/she/it asks

Here the subject is kaupandinn, so spyr means asks.

In Icelandic, spyrja um + accusative is a common way to say ask about something.

So:

• spyr um verðið = asks about the price

Without um, the verb structure changes and usually means asking a person, or asking a question more directly. In this sentence, the idea is specifically asking about a topic, so um is the natural choice.

Kaupandinn is in the nominative case because it is the subject of the sentence. It is the person doing the action of asking.

In Kaupandinn spyr um verðið:

• kaupandinn = subject, so nominative
• verðið = object of the preposition um, so accusative

Yes, it matters, because gender affects articles and inflection.

• kaupandi is masculine
• verð is neuter

That is part of why their definite forms look different:

• masculine: kaupandinn
• neuter: verðið

In Icelandic, noun gender is important because it affects articles, adjectives, pronouns, and case endings.

Yes, this sentence has the normal subject-verb-prepositional phrase order, which is very similar to English:

• Kaupandinn = the buyer
• spyr = asks
• um verðið = about the price

So the structure is basically The buyer asks about the price.

Icelandic can change word order more freely than English in some contexts, but this version is the neutral, standard one.

A rough guide for an English speaker is:

• Kaupandinn: first syllable stressed; kau- has a diphthong
• spyr: the y is not pronounced like English y; it is a vowel sound
• verðið: ð is like the th in this

A few useful pronunciation points:

• Icelandic almost always stresses the first syllable.
• ð is usually a soft voiced th sound.
• ll, nn, rn, and other consonant combinations can sound different from what an English speaker expects, so listening to native audio helps a lot.

Yes. Icelandic present tense often covers both the simple present and the present progressive, depending on context.

So Kaupandinn spyr um verðið can mean:

• The buyer asks about the price
• The buyer is asking about the price

Usually the surrounding context tells you which English translation fits best.