Gamla rafhlaðan var gagnslaus, svo síminn slokknaði strax aftur.

Because gamla rafhlaðan means the old battery, not just an old battery.

Two things are happening:

• rafhlaðan is the definite form: the battery
• the adjective gamall (old) has to agree with the noun and takes the weak form when the noun is definite

So:

• gömul rafhlaða = an old battery
• gamla rafhlaðan = the old battery

This is a very common pattern in Icelandic:

• gamall maður = an old man
• gamli maðurinn = the old man

That ending is the definite article attached to the noun.

In Icelandic, the is usually not a separate word. Instead, it is added to the end of the noun.

So:

• rafhlaða = battery
• rafhlaðan = the battery

Likewise:

• sími = phone
• síminn = the phone

This attached article is one of the first big differences English speakers notice in Icelandic.

Because gagnslaus is a predicate adjective after var (was), and predicate adjectives usually appear in the strong form.

In this sentence:

• rafhlaðan is feminine singular
• gagnslaus agrees with it

But with this adjective, the strong nominative singular masculine and feminine forms are the same:

• masculine: gagnslaus
• feminine: gagnslaus
• neuter: gagnslaust

So even though it refers to a feminine noun, gagnslaus is correct.

This often surprises learners because gamla earlier in the sentence is weak feminine, while gagnslaus is strong feminine. That difference comes from their grammatical roles:

• gamla is directly before a definite noun
• gagnslaus comes after vera as a description of the subject

Var is the past singular form of vera, which means to be.

So:

• er = is
• var = was

In this sentence, rafhlaðan var gagnslaus means the battery was useless.

Vera is an irregular verb, so its forms need to be memorized.

Here svo means so, introducing a result or consequence.

So the structure is:

• Gamla rafhlaðan var gagnslaus
• svo síminn slokknaði strax aftur

In other words, the second clause is the result of the first one.

Be careful, because svo can mean different things in Icelandic depending on context, such as:

• so
• then
• very/so in some expressions

But here the meaning is the conjunction so.

Because síminn is the subject of the second clause, so it is in the nominative case.

The second clause is:

• síminn slokknaði

That means the phone turned off / went off

Since the phone is the thing doing the action grammatically, it stays in the nominative:

• sími = phone
• síminn = the phone

If it were an object instead, you might see a different case form.

It comes from slokkna, which means to go out, to switch off, or to stop being on.

So:

• slokkna = to go out / turn off by itself
• slokknaði = went out / turned off

This is the past tense.

A very useful contrast is:

• slokkna = intransitive
  something goes off
• slökkva = transitive
  someone switches something off

For example:

• Síminn slokknaði. = The phone turned off.
• Ég slökkti á símanum. = I switched off the phone.

That distinction is important in Icelandic.

Because slokkna is an intransitive verb.

It describes the device going off, without naming a person who caused it. In this sentence, the idea is that the phone shut down because the battery was useless.

If you wanted to say that a person turned it off, you would normally use slökkva instead.

So the choice of slokknaði fits the meaning very naturally.

They add two different pieces of meaning:

• strax = immediately / right away
• aftur = again

Together, strax aftur means something like right away again.

The sense is that the phone came on or tried to function, but then quickly turned off again.

If you remove one word, the meaning changes:

• slokknaði strax = turned off immediately
• slokknaði aftur = turned off again
• slokknaði strax aftur = turned off again immediately

Because after svo used like so, the next clause often behaves like a normal main clause.

In a normal Icelandic main clause, the finite verb is typically in second position. Here the subject comes first:

• síminn = 1st element
• slokknaði = 2nd element

So:

• svo síminn slokknaði strax aftur

is perfectly natural.

You may also see svo slokknaði síminn..., but that would put more emphasis on the event itself, more like and then the phone turned off... The version in your sentence is neutral and straightforward.

Yes. It is a very useful adjective, and its structure can help you remember it.

• gagn relates to use or benefit
• -laus means without, similar to English -less

So gagnslaus is literally something like useless / without use.

You can compare it with other Icelandic adjectives ending in -laus, such as:

• vonlaus = hopeless
• ráðalaus = helpless / at a loss
• tilgangslaus = pointless

This ending is very productive and worth learning.

Because the sentence contains two coordinated clauses:

• Gamla rafhlaðan var gagnslaus
• svo síminn slokknaði strax aftur

The comma helps separate the two parts clearly, much like in English.

Punctuation can vary a bit by style, but this comma is completely natural and helps show that the second clause is the consequence of the first.