Hún sendi mér minnisblaðið aftur, því töflureiknirinn opnaðist ekki á tölvunni minni.

Here sendi is past tense, because the subject is hún = she.

A useful thing to know is that sendi is one of those forms that can look ambiguous if you see it alone:

• ég sendi = I send
• hún sendi = she sent

So the subject tells you how to understand it. In this sentence, hún sendi clearly means she sent.

Because senda normally takes:

• a recipient in the dative
• a thing sent in the accusative

So the pattern is:

• senda einhverjum eitthvað = send someone something

Here:

• mér = to me / me as the recipient, so it is dative
• minnisblaðið = the thing being sent

That is why mér is correct, not mig.

The base noun is minnisblað, which is a neuter noun.

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

• minnisblað = memo / note
• minnisblaðið = the memo / the note

In this sentence it is the direct object of sendi. For many neuter nouns, the nominative and accusative singular look the same, so minnisblaðið is the expected form here.

Here aftur most naturally means back.

So:

• senda aftur = send back

In other sentences, aftur can also mean again, so context matters. But with a verb like senda, learners will often understand it as back unless the context clearly points to repetition.

That word order is very natural in Icelandic.

The first clause is basically:

• Hún — subject
• sendi — verb
• mér — indirect object
• minnisblaðið — direct object
• aftur — adverb

So the structure is:

• Hún sendi mér minnisblaðið aftur

Icelandic adverb placement can be somewhat flexible, but aftur after the object sounds normal here, especially in the common phrase senda eitthvað aftur.

Because því introduces an explanatory clause here, roughly like because or for in English.

So the sentence is divided into two parts:

• Hún sendi mér minnisblaðið aftur
• því töflureiknirinn opnaðist ekki á tölvunni minni

Writing a comma before því is normal in this kind of sentence.

Here því means because / since / for.

It introduces the reason for what happened in the first clause.

A very common alternative is af því að, which also means because:

• ..., því ...
• ..., af því að ...

The difference is mostly one of style and sentence structure:

• því often sounds a bit more concise and sometimes a bit more formal or written
• af því að is extremely common in everyday language

In this sentence, því works perfectly naturally.

Because after því, Icelandic usually continues with a full clause that keeps normal clause order.

So:

• töflureiknirinn = subject
• opnaðist = finite verb

That gives:

• því töflureiknirinn opnaðist ekki ...

A good way to think of it is that því links on a new clause explaining the reason, and that clause behaves much like an independent clause in its word order.

The base noun is töflureiknir, which is a masculine noun.

The ending -inn is the attached definite article, so:

• töflureiknir = spreadsheet
• töflureiknirinn = the spreadsheet

Here it is the subject of opnaðist, so the form is nominative singular definite.

Because opnaðist is the form used when something opened or failed to open by itself, without stating an outside agent.

Compare:

• Hún opnaði skrána. = She opened the file.
• Skráin opnaðist ekki. = The file didn’t open.

So in your sentence:

• töflureiknirinn opnaðist ekki = the spreadsheet didn’t open

The -st form often has a middle-voice or passive-like sense in Icelandic. It is very common in sentences like this.

Because in Icelandic, the negative adverb ekki usually comes after the finite verb.

So:

• opnaðist ekki = did not open

That placement is completely standard. English puts not after an auxiliary verb, but Icelandic often just puts ekki after the finite verb itself.

Because á takes the dative when it means location on / in / on the device, rather than motion toward somewhere.

Here the meaning is location:

• the spreadsheet did not open on my computer

So tölva has to be in the dative singular:

• tölva = computer
• tölvunni = on the computer / in the computer context here

Then minni agrees with it:

• tölvunni minni = my computer in the dative

This is also a very common Icelandic pattern with possessed nouns:

• tölvan mín = my computer
• á tölvunni minni = on my computer

A lot of the endings in this sentence come from gender, case, and definiteness.

The three main nouns are:

• minnisblað — neuter
• töflureiknir — masculine
• tölva — feminine

That helps explain the different forms:

• minnisblaðið: neuter definite singular
• töflureiknirinn: masculine definite singular
• tölvunni: feminine dative definite singular
• minni: possessive agreeing with feminine dative singular

So even if the sentence looks full of endings, they are not random. They show:

• what role the noun has in the sentence
• whether it is definite
• what gender it belongs to