Learn Hardcore Finnish: Kun netti toimii taas, lähetän viestin ystävälleni. - When the internet works again, I will send a message to my friend.

Finnish commonly uses the present tense in time clauses introduced by kun to talk about future situations. So Kun netti toimii taas literally looks like When the internet works again, but it naturally means When the internet is working again (in the future).
English often uses when ... works too, but Finnish uses this pattern very consistently (rather than a special future tense, which Finnish doesn’t really have).

What exactly does kun mean here, and how is it different from jos?

Here kun means when (a time relationship that’s expected or treated as a matter of time).
jos means if (a condition that may or may not happen).

So:

Kun netti toimii taas, lähetän viestin. = When the internet works again, I’ll send a message. (expected sooner or later)
Jos netti toimii, lähetän viestin. = If the internet works, I’ll send a message. (more conditional/uncertain)

Why is there a comma after taas?

Because Finnish separates a clause that begins with kun (a subordinate clause) from the main clause with a comma.
Structure:

Subordinate clause: Kun netti toimii taas,
Main clause: lähetän viestin ystävälleni.

This comma rule is very regular in Finnish.

What does taas add, and where can it go in the sentence?

taas means something like again / once more / back (to working). It implies the internet wasn’t working before, and now it returns to normal.

Placement is fairly flexible, with slight emphasis shifts:

Kun netti toimii taas, ... = when the internet works again (neutral)
Kun netti taas toimii, ... = when the internet, again, works (more emphasis on “again”)
Kun taas netti toimii, ... can be possible but can start to sound like taas is linking/contrasting ideas depending on context.

Why is netti used instead of internet?

netti is the common everyday word for the internet in Finnish (colloquial/neutral). internet is also used and understood, but netti is very natural in speech and informal writing.

What does toimia mean here? Is it “to function” or “to work”?

toimia literally means to function / to operate, and in this context it’s exactly the idiomatic way to say (the internet) works.
So netti toimii = the internet is working / the internet works.

Why is it lähetän (present tense) if the meaning is “I will send”?

Finnish often uses the present tense to express near/real future actions, especially when the time is implied by context (here: the kun clause). There isn’t a dedicated future tense like English will, so lähetän comfortably covers I send / I will send depending on context.

Why is viestin in that form—why not viestiä?

viestin is the total object form (often called accusative/genitive-looking), used when the action is seen as complete: send (the/a) message as a finished event.

viestiä (partitive) would suggest an incomplete/ongoing or less bounded action, like send some message(s) / message (in general) / be sending a message depending on context.
So here:

lähetän viestin = I’ll send a (complete) message
lähetän viestiä = I’ll be sending messages / I’ll send some message (less bounded)

What case is ystävälleni, and what does it mean?

ystävälleni breaks down as:

ystävä = friend
-lle = allative case = to / onto
-ni = possessive suffix = my

So ystävälleni means to my friend.

Could I also say lähetän viestin minun ystävälleni? What changes?

Yes. lähetän viestin minun ystävälleni is grammatical, but usually heavier than needed because -ni already marks possession.

Common options:

ystävälleni = to my friend (very common, compact)
minun ystävälleni = to my friend (emphasizes my, but often still used without the suffix in some styles)
minun ystävälleni
- -ni together (minun ystävälleni) doesn’t normally take -ni because the possessive is already expressed with minun; the neat Finnish way is usually one or the other, and in this sentence ystävälleni is the most natural.