Aðalskoðunin byrjar í dag.

Because Icelandic very often makes compound nouns by joining words together.

Here:

• aðal- = main, principal
• skoðun = inspection, examination, review

So aðalskoðun is a single compound noun meaning something like main inspection. Icelandic prefers this one-word compound rather than writing it as two separate words.

Aðal- means main, principal, or chief.

It is commonly used as the first part of compounds, for example:

• aðalinngangur = main entrance
• aðalatriði = main point
• aðalskoðun = main inspection

So in this sentence, aðal- modifies skoðun and tells you this is the primary or main inspection.

The ending -in is the definite article attached to the noun. In English, this is usually expressed with the separate word the.

So:

• aðalskoðun = main inspection
• aðalskoðunin = the main inspection

This is very normal in Icelandic. Instead of putting a separate word before the noun, Icelandic often adds the article to the end of the noun.

Aðalskoðunin is:

• singular
• feminine
• nominative
• definite

The base noun skoðun is feminine, and the whole compound keeps that gender.

It is in the nominative case here because it is the subject of the sentence — it is the thing that starts.

Byrjar is the present tense, 3rd person singular form of the verb byrja = to begin / to start.

So:

• ég byrja = I begin
• þú byrjar = you begin
• hún/hann/það byrjar = she/he/it begins

In this sentence, the subject Aðalskoðunin is singular, so the verb is also singular: byrjar.

Í dag is a very common fixed Icelandic expression meaning today.

Word-for-word, it looks like in day, but you should learn it as a set phrase:

• í dag = today
• á morgun = tomorrow
• í gær = yesterday

Even if the literal structure feels unusual from an English point of view, í dag is simply the normal everyday way to say today in Icelandic.

Here dag is in the accusative singular.

The noun is:

• dagur = day
• dag = accusative singular

In í dag, this accusative form is part of a fixed time expression. So for a learner, the most useful thing is to remember the whole phrase í dag as one unit meaning today.

Yes. Icelandic word order is somewhat flexible, but the verb often stays in second position in main clauses.

The neutral order is:

• Aðalskoðunin byrjar í dag.

You can also front the time expression:

• Í dag byrjar aðalskoðunin.

That version puts more emphasis on today.

A useful rule for learners is: if you move something to the front in Icelandic, the finite verb usually still comes second.

You should look up aðalskoðun first.

That is the form without the attached definite article:

• aðalskoðunin = the main inspection
• aðalskoðun = main inspection

If you still cannot find it, break it into parts:

• aðal-
• skoðun

This is a very useful strategy in Icelandic, because compounds are extremely common.

It is capitalized simply because it is the first word of the sentence.

Icelandic does not capitalize ordinary nouns the way German does. So in the middle of a sentence, you would normally write aðalskoðunin with a lowercase a, unless there were some special reason not to.