Þetta var erfið ákvörðun, en nú er ég viss um hana.

Þetta is the neuter form of this, and Icelandic often uses it the same way English uses this to refer to a whole situation or statement:

• Þetta var erfið ákvörðun = This was a difficult decision

Here, þetta does not directly modify ákvörðun. It points to the situation as a whole.

If you wanted to say this decision, you would use:

• þessi ákvörðun

So compare:

• Þetta var erfið ákvörðun = This was a difficult decision
• Þessi ákvörðun var erfið = This decision was difficult

Because ákvörðun is a feminine singular noun in the nominative, and the adjective has to agree with it.

• ákvörðun = decision
• gender: feminine
• number: singular
• case here: nominative (because it is the predicate noun phrase after var)

So the adjective erfiður (difficult) appears in the feminine nominative singular form:

• masculine: erfiður
• feminine: erfið
• neuter: erfitt

That is why you get:

• erfið ákvörðun

It is in the nominative.

In the clause:

• Þetta var erfið ákvörðun

the phrase erfið ákvörðun is a predicate complement after the verb vera (to be). In Icelandic, predicate nouns and adjectives with vera are usually in the nominative.

So:

• ákvörðun = nominative singular
• erfið agrees with it in nominative singular feminine

Hana refers back to ákvörðun.

Since ákvörðun is a feminine noun, the pronoun that refers to it must also be feminine:

• ákvörðun = feminine
• hana = her/it in the feminine accusative singular

Even though English uses it for a thing like decision, Icelandic still uses grammatical gender, so the pronoun must match the noun’s gender.

So:

• um hana = about it / literally about her

Here, hana is best translated as it, not her, because it refers to a decision.

Because the preposition um normally takes the accusative case when it means about or around.

So after um, the pronoun must be accusative:

• nominative: hún = she/it
• accusative: hana

That is why Icelandic says:

• viss um hana = sure about it

This is something you simply have to learn with the preposition: um + accusative.

Viss um means sure about, certain about, or confident about.

It is a very common expression in Icelandic:

• vera viss um eitthvað = to be sure about something

Examples:

• Ég er viss um þetta. = I am sure about this.
• Ertu viss um svarið? = Are you sure about the answer?

So in your sentence:

• nú er ég viss um hana
• now I am sure about it

This is an idiomatic combination, so it is good to learn viss um as a unit.

Because the sentence contrasts a past situation with a present one:

• Þetta var erfið ákvörðun = This was a difficult decision
• en nú er ég viss um hana = but now I am sure about it

So:

• var = past tense of vera (was)
• er = present tense of vera (am/is)

The speaker is saying:

1. the decision was difficult in the past, when it had to be made
2. now, in the present, they are sure about it

Because Icelandic usually puts the finite verb in the second position in main clauses. This is often called V2 word order.

In the second clause:

• en nú er ég viss um hana

the first element is nú (now), so the verb er comes next.

Structure:

• en = but
• nú = now
• er = am/is
• ég = I

If the sentence began with ég instead, then the verb would still come second:

• Ég er nú viss um hana.

That version is also possible, but it has a slightly different emphasis. The original sentence emphasizes now.

Yes. Icelandic can repeat the noun, but using the pronoun is more natural if the reference is already clear.

So you could say:

• Þetta var erfið ákvörðun, en nú er ég viss um ákvörðunina.

But that sounds more repetitive than:

• ... en nú er ég viss um hana.

Using hana is normal because the listener already knows that it refers to ákvörðun.

These are two very important Icelandic letters:

• þ is pronounced like th in thing
• ð is pronounced like th in this

From this sentence:

• Þetta begins with þ: roughly THET-ta
• erfið contains ð at the end: roughly ER-vith or ER-vith/ER-vithth, depending on speech style

A few quick pronunciation notes:

• þetta ≈ THET-ta
• var ≈ var
• erfið ≈ ER-vith
• ákvörðun ≈ OWK-vur-thun
• nú ≈ noo
• ég ≈ yeg / yehg
• viss ≈ viss
• um hana ≈ um HA-na

Exact pronunciation varies a little by speaker, but the main thing is to keep þ and ð separate:

• þ = voiceless th
• ð = voiced th

Yes, the adjective form would change.

In your sentence, erfið ákvörðun is indefinite:

• a difficult decision

If you wanted the difficult decision, Icelandic would normally use the weak adjective form plus the definite noun:

• erfiða ákvörðunin

So compare:

• erfið ákvörðun = a difficult decision
• erfiða ákvörðunin = the difficult decision

This is a common Icelandic pattern:

• indefinite noun -> strong adjective
• definite noun -> weak adjective

That is why erfið is correct in your original sentence.