Fyrsta umræðuefnið á fundinum var kostnaðurinn, en í fundargerðinni stóð líka hvaða atriði við ræddum seinna.

Because umræðuefnið is a neuter singular noun in the nominative, and fyrsta has to match it.

Also, fyrsti/fyrsta/fyrstu is an ordinal adjective, and here it is in the weak form, which is what you normally get when the noun is definite. Since the noun is umræðuefnið = the discussion topic, the adjective becomes fyrsta:

• fyrsta umræðuefnið = the first discussion topic

Umræðuefnið is the noun umræðuefni, which means discussion topic / subject for discussion. It is a neuter noun.

The ending -ið is the suffixed definite article, so:

• umræðuefni = a discussion topic
• umræðuefnið = the discussion topic

This is very common in Icelandic: instead of a separate word for the, Icelandic usually adds the article to the end of the noun.

Because á here means at/on in the sense of location, not movement. With many Icelandic prepositions, the case changes depending on whether you mean:

• location / being somewhere → often dative
• movement toward somewhere → often accusative

Here the meaning is at the meeting, so Icelandic uses the dative:

• fundur = meeting
• fundinum = the meeting in the dative singular

So á fundinum means at the meeting.

Because after vera (to be), Icelandic normally uses the nominative for the complement as well.

So in:

• Fyrsta umræðuefnið ... var kostnaðurinn

both umræðuefnið and kostnaðurinn are nominative. This is similar to English The problem was the cost.

The -inn ending is again the suffixed definite article:

• kostnaður = cost
• kostnaðurinn = the cost

The verb standa literally means to stand, but it is also commonly used about text to mean to stand written, to be written, or to be stated.

So:

• í fundargerðinni stóð ...
  means something like
• in the minutes it said ...
• it was written in the minutes ...

The past tense of standa is stóð.

This is because Icelandic is a verb-second language in main clauses.

That means the finite verb usually comes in the second position. If you put another element first, such as a prepositional phrase, the verb still comes second:

• Í fundargerðinni = first element
• stóð = second element

So the structure is natural Icelandic main-clause word order.

Hvaða means which / what, and here it introduces an indirect question:

• hvaða atriði við ræddum seinna
  = which points/issues we discussed later

So this is not a simple relative clause with sem. It is more like embedded question content: the minutes stated which items were discussed later.

Also, after hvaða, Icelandic normally uses the noun without the definite article:

• hvaða atriði
  not hvaða atriðin

Because atriði is a neuter noun whose nominative/accusative singular and plural can have the same form.

So atriði can mean either:

• an item / a point
• items / points

You tell which one is meant from the context. Here it is naturally understood as plural: which points/issues.

Ræddum is the past tense, 1st person plural form of ræða (to discuss).

It matches við = we:

• við ræddum = we discussed

So the ending shows who did the action.

Seinna is an adverb meaning later. It modifies the verb ræddum:

• við ræddum seinna = we discussed later

It tells you when the discussion happened relative to something earlier in the meeting.

The basic noun is fundargerð, a feminine noun meaning minutes / meeting record.

Here it appears as fundargerðinni, which is:

• dative singular
• definite

That is because it follows í in a location sense:

• í fundargerðinni = in the minutes

So this is another example of a preposition requiring the dative when it expresses location.

Líka means also.

Its position is quite natural in Icelandic after the finite verb in a main clause, especially when the sentence then continues with a longer content phrase:

• í fundargerðinni stóð líka ...

So the sense is:

• it also said in the minutes ...

You could think of líka as adding another piece of information to the first clause.