Yfirmaðurinn sendi fundarboð í morgun, en ég sá það ekki fyrr en í hádeginu.

Because Icelandic usually puts the definite article on the end of the noun.

• yfirmaður = boss / supervisor
• yfirmaðurinn = the boss / the supervisor

Here it is the subject of the sentence, so it is in the nominative singular.

Sendi is the past tense of senda (to send).

In this sentence it means sent. Icelandic past tense does not need an extra helping verb like English sometimes does. So:

• yfirmaðurinn sendi = the boss sent

This is the normal 3rd person singular past form, but in Icelandic the same past singular form is also used with ég:

• ég sendi = I sent
• hann/hún/það sendi = he/she/it sent

Fundarboð is a compound noun.

It is built from:

• fundur = meeting
• fundar- = the genitive form used inside the compound
• boð = invitation / notice / message

So fundarboð literally means something like meeting-invitation or meeting notice, and in natural English it is usually meeting invitation or meeting notice.

Using the genitive form in compounds is very common in Icelandic.

Because Icelandic has no separate word for a/an.

A bare singular noun like fundarboð can mean:

• a meeting invitation
• a meeting notice

If the speaker wanted to say the meeting invitation, they would normally say fundarboðið.

So here fundarboð is indefinite, like English a meeting invitation.

That is a very good question, because boð looks the same in nominative/accusative singular and plural.

The clue comes later:

• það = it

Since the speaker says það, the noun is being treated as singular neuter. If it were plural, you would expect a plural pronoun instead.

Also, það is used because boð is a neuter noun, and Icelandic pronouns must agree with the noun’s grammatical gender and number.

Sá is the past tense of sjá (to see).

So:

• sjá = to see
• sá = saw

This is an irregular verb, so it is worth memorizing separately. In the sentence:

• ég sá það = I saw it

There is no separate word for did here, because Icelandic does not use English-style do-support.

English says:

• I did not see it

Icelandic simply uses the past tense verb plus negation:

• ég sá það ekki
• literally: I saw it not
• naturally: I didn’t see it

So sá already carries the past meaning, and ekki adds the negation.

Because short object pronouns like það often come before sentence adverbs such as ekki.

So this order is very natural:

• ég sá það ekki = I didn’t see it

With a full noun phrase, Icelandic more often puts ekki before the noun:

• ég sá ekki fundarboðið = I didn’t see the meeting invitation

So the position of það is connected to the fact that it is a short pronoun.

Ekki fyrr en is a very common Icelandic expression meaning not until.

So:

• ég sá það ekki fyrr en í hádeginu
• = I didn’t see it until noon / lunchtime

Word-for-word, it is closer to not earlier than, but you should usually understand and translate it as not until.

These are both time expressions, but they work a little differently.

• í morgun = this morning
• í hádeginu = at noon / at lunchtime

Í morgun is basically a fixed expression that learners should memorize as a chunk. It normally appears without the definite article.

Í hádeginu contains the definite form of hádegi (noon), so the ending shows that article.

This kind of difference is common in Icelandic time phrases: some are very idiomatic and are best learned as set expressions.

Normally, í can take:

• accusative for motion into something
• dative for location or state

But with time expressions, things can become idiomatic.

• í hádeginu is a dative time expression
• í morgun is a fixed adverbial phrase that is best learned as a whole

So the most practical approach is:

• memorize í morgun = this morning
• memorize í hádeginu = at noon / at lunchtime

rather than trying to force both into the same simple rule.

Yes. En is the ordinary coordinating conjunction meaning but.

It joins two main clauses:

• Yfirmaðurinn sendi fundarboð í morgun
• en ég sá það ekki fyrr en í hádeginu

So it works very much like English but in this sentence.