Ef skjátextinn hverfur, þarf hún að breyta stillingunni í forritinu, og stundum þarf hún líka að athuga hljóðnemann.

Skjátextinn means the subtitle / the on-screen text, while skjátexti would mean a subtitle / on-screen text in a more general or indefinite sense.

Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the.

• skjátexti = subtitle / on-screen text
• skjátextinn = the subtitle / the on-screen text

Here it makes sense because the sentence is talking about a specific thing disappearing.

Because the subject skjátextinn is singular, and hverfur is the 3rd person singular present form of hverfa.

So:

• ég hverf
• þú hverfur
• hann/hún/það hverfur

In this sentence:

• skjátextinn hverfur = the subtitle disappears

Even though skjátextinn is not a person, Icelandic still uses the same 3rd person singular verb form.

Ef means if and introduces a subordinate clause:

• Ef skjátextinn hverfur = If the subtitle disappears

After that comes the main clause:

• þarf hún að breyta stillingunni í forritinu

A very important Icelandic word-order point is that when a sentence starts with a subordinate clause, the main clause usually has the verb before the subject:

• Ef ... , þarf hún ...
• not Ef ... , hún þarf ...

This is part of Icelandic’s verb-second pattern in main clauses.

Because the sentence begins with the Ef clause.

In a simple main clause, you would normally say:

• Hún þarf að breyta stillingunni.
  = She needs to change the setting.

But when something else comes first, such as a subordinate clause, the finite verb usually comes before the subject in the main clause:

• Ef skjátextinn hverfur, þarf hún að breyta stillingunni.

So the word order changes because Ef skjátextinn hverfur occupies the first position.

The verb þurfa means to need, and it is commonly followed by að plus an infinitive.

So:

• þarf að breyta = needs to change
• literally: needs to change

This is a very common pattern in Icelandic:

• Ég þarf að fara. = I need to go.
• Hún þarf að lesa. = She needs to read.

So in your sentence:

• þarf hún að breyta = she needs to change

Because the verb breyta takes the dative case for the thing being changed.

This is something English speakers often have to memorize: Icelandic verbs often require specific cases for their objects.

Here:

• stilling = setting
• stillingunni = the setting in the dative singular

So:

• breyta einhverju = change something
  literally, the something is in dative

Examples:

• breyta planinu = change the plan
• breyta kerfinu = change the system
• breyta stillingunni = change the setting

So the ending is there because breyta governs the dative.

Because í can take either the accusative or the dative, depending on meaning.

• í + dative usually means in / inside / within a place, with no movement into it
• í + accusative usually means movement into something

Here the meaning is location:

• í forritinu = in the program/app

There is no movement into the app, just location within it, so Icelandic uses the dative.

• forritið = the program/app
• forritinu = in the program/app (dative singular with the definite article)

Hljóðnemann is the definite accusative singular of hljóðnemi (microphone).

The verb athuga usually takes a direct object in the accusative, so:

• athuga hljóðnemann = check the microphone

Compare:

• hljóðnemi = a microphone
• hljóðnemann = the microphone (accusative singular)

So the ending changes because this noun is the direct object of athuga.

Líka means also / too, and its position is fairly natural here:

• stundum þarf hún líka að athuga hljóðnemann

This means something like:

• sometimes she also needs to check the microphone

The word líka often comes near the part it adds to. Here it signals that checking the microphone is an additional thing she may need to do, besides changing the setting.

Other placements are sometimes possible, but this one is very normal and idiomatic.

Stundum means sometimes. It is an adverb of frequency.

So:

• og stundum þarf hún líka að athuga hljóðnemann = and sometimes she also needs to check the microphone

It tells you that this second action does not happen every time.

Yes. Og is the ordinary Icelandic word for and.

Here it joins two coordinated parts:

1. Ef skjátextinn hverfur, þarf hún að breyta stillingunni í forritinu
2. og stundum þarf hún líka að athuga hljóðnemann

So it works very much like English and.

Yes:

• Ef = if
• skjátextinn = the subtitle / the on-screen text
• hverfur = disappears
• þarf hún = she needs
• að breyta = to change
• stillingunni = the setting
• í forritinu = in the program/app
• og = and
• stundum = sometimes
• þarf hún líka = she also needs
• að athuga = to check
• hljóðnemann = the microphone

This kind of chunking is very useful in Icelandic, especially because case endings and word order can make full sentences feel dense at first.