Ég misskildi skilaboðin hennar, og misskilningurinn gerði kvöldið erfiðara.

Misskildi is the past tense of misskilja, meaning to misunderstand.

It is built from:

• skilja = to understand
• prefix mis- = wrongly, badly, mistakenly

So misskilja literally means something like understand wrongly.

They come from the same root, but they are different parts of speech:

• misskildi = a verb: misunderstood
• misskilningurinn = a noun: the misunderstanding

This is similar to English:

• I misunderstood
• the misunderstanding

Because -in here is the attached definite article, so skilaboðin means the message(s) or the messages.

Compare:

• skilaboð = message/messages
• skilaboðin = the message/messages

A useful thing to know is that skilaboð is very often used in a plural-looking form in Icelandic, even where English might simply say message.

In Icelandic, possessive expressions with personal pronouns are very commonly placed after the noun:

• skilaboðin hennar = her message(s)
• bíllinn hans = his car
• barnið þeirra = their child

So this word order is very natural. English usually puts her before the noun, but Icelandic often puts hennar after it.

Hennar is the genitive form of hún (she), and it is used to mean her.

So:

• hún = she
• hennar = her

In this sentence, hennar shows possession: the message belonged to or came from her.

Misskilningur means misunderstanding.
Misskilningurinn means the misunderstanding.

The ending -inn is the definite article (the), but with nouns like misskilningur, the form becomes -urinn:

• misskilningur = misunderstanding
• misskilningurinn = the misunderstanding

Here it is the subject of the second clause.

Gerði is the past tense of gera, which means to do, to make.

So:

• gera = to make
• gerði = made

In this sentence, it means made in the sense of caused to become:

• gerði kvöldið erfiðara = made the evening more difficult

Kvöldið means the evening.

It comes from:

• kvöld = evening
• kvöldið = the evening

It is the direct object of gerði. With a neuter noun like kvöld, the nominative and accusative definite forms look the same, so you do not see a special extra ending for the object case here.

Because erfiðara is the comparative form: more difficult.

Compare:

• erfitt = difficult
• erfiðara = more difficult

So:

• gerði kvöldið erfitt = made the evening difficult
• gerði kvöldið erfiðara = made the evening more difficult

The adjective agrees with kvöldið, which is neuter singular, so erfiðara is the correct form here.

This is a very common Icelandic pattern:

gera + object + adjective

It means make something become X.

Examples:

• Þetta gerir lífið auðveldara. = This makes life easier.
• Veðrið gerði ferðina erfiðari. = The weather made the trip more difficult.

So misskilningurinn gerði kvöldið erfiðara means that the misunderstanding caused the evening to become harder or more difficult.

Yes, it is acceptable here because og joins two full clauses:

• Ég misskildi skilaboðin hennar
• misskilningurinn gerði kvöldið erfiðara

In practice, comma use before og can vary somewhat by style, but with two independent clauses like this, the comma is not surprising.