Honum finnst veðrið gott í dag.

Because honum is the dative form of hann (he/him).

The verb finnast often works with an experiencer in the dative. In this sentence, honum means to him rather than just he.

So the structure is roughly:

• honum = to him
• finnst = seems / feels / is found
• veðrið gott = the weather good

A very literal English rendering would be:

• To him, the weather seems good today.

That is why nominative hann is not used here.

Finnst is the present singular form of the verb finnast.

In sentences like this, finnast is commonly used to express:

• to think
• to find
• to feel
• to seem

So Honum finnst veðrið gott í dag means something like:

• He thinks the weather is good today
• He finds the weather nice today

It does not literally mean physical touching or ordinary finding in the sense of locating something. It is about a person’s impression or opinion.

Because finnst already carries the main idea of seems / feels / is found to be.

English often says:

• He thinks the weather is good today

But Icelandic can express this with one main verb, finnast, plus an adjective:

• Honum finnst veðrið gott í dag

So Icelandic does not need a separate er (is) here.

Veðrið is nominative singular neuter definite of veður (weather).

Breaking it down:

• veður = weather
• veðrið = the weather

The ending -ið is the neuter singular definite article attached to the noun.

So this is not just weather in a general sense, but the weather.

Because gott must agree with veðrið.

Here is the key point:

• veðrið is neuter singular
• so the adjective must also be neuter singular
• the neuter singular form of góður is gott

So:

• masculine singular: góður
• feminine singular: góð
• neuter singular: gott

Since veðrið is neuter, gott is the correct form.

Yes. Gott is a predicate adjective describing veðrið.

The sentence is not saying:

• good weather as one noun phrase

It is saying:

• the weather
  • is considered/seems good

So veðrið is the thing being talked about, and gott tells you the opinion about it.

That is why the phrase behaves more like:

• the weather [is] good

than like a simple noun phrase such as gott veður (good weather).

A helpful literal breakdown is:

• Honum = to him
• finnst = seems / feels / is found
• veðrið = the weather
• gott = good
• í dag = today

So a very literal translation would be:

• To him, the weather seems good today.

That literal structure helps explain why Icelandic uses dative honum.

Yes. Icelandic word order is somewhat flexible, especially when you move time expressions or emphasize different parts.

This sentence can stay as:

• Honum finnst veðrið gott í dag.

But you could also see things like:

• Í dag finnst honum veðrið gott.

That still means essentially the same thing: He thinks the weather is good today.

The original version is very natural, but Icelandic often allows movement of adverbs like í dag.

Not exactly.

Honum finnst ... is very common for giving a personal impression, opinion, or feeling:

• Honum finnst veðrið gott.
• He thinks / feels the weather is good.

Hann heldur að ... is more like:

• He thinks that ...
• often more about belief or assumption

For example:

• Hann heldur að það muni rigna.
• He thinks it will rain.

So finnast is often more natural for personal judgments like good, boring, funny, beautiful, and so on.

Sometimes in natural English, yes, but it is not the most exact match.

Honum finnst veðrið gott í dag most directly means:

• He thinks the weather is good today
• He finds the weather nice today

If you say He likes the weather today, that gives a similar overall idea, but finnast is really about someone’s evaluation or impression.

A closer Icelandic verb for to like is often líka, as in:

• Honum líkar veðrið.
• He likes the weather.

So the two ideas are related, but not identical.

Í dag literally means in day, but idiomatically it means today.

It is just the normal Icelandic expression:

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

So even though English uses one word, Icelandic often uses a short phrase.

Here are the dictionary forms:

• honum → hann (he)
• finnst → finnast
• veðrið → veður
• gott → góður
• í dag → fixed expression meaning today

This is useful because Icelandic words often appear in inflected forms that look different from the dictionary entry.

In a practical learner sense, yes, it is often treated that way because the person having the opinion is not in the nominative.

The important learner pattern is:

• Mér finnst ... = I think / I find ...
• Þér finnst ... = you think / you find ...
• Honum finnst ... = he thinks / he finds ...
• Henni finnst ... = she thinks / she finds ...

So even if the sentence has a nominative element like veðrið, the person experiencing the opinion is in the dative. That is the main thing to learn and remember.