Learn Hardcore Romanian: Când prietenii sună, bunicul așteaptă la ușă și le spune salut. - When the friends call on the doorbell, grandfather waits at the door and says hello to them.

In Romanian, when a sentence starts with a time clause introduced by când (or other conjunctions like dacă, deoarece, etc.), you normally put a comma between that clause and the main clause.

Când prietenii sună, – subordinate clause (When the friends call)
bunicul așteaptă la ușă și le spune salut. – main clause (grandpa waits at the door and says hello to them)

If you reverse the order, the comma usually disappears:

Bunicul așteaptă la ușă și le spune salut când prietenii sună.
(No comma needed here in normal usage.)

Why is it prietenii and not prieteni?

Romanian marks the definite article at the end of the noun, not before it like English.

prieteni = friends (indefinite, some friends)
prietenii = the friends (definite)

Here, the meaning is the friends (specific group), so Romanian uses the definite form prietenii.

Formally:

singular: prieten (friend), prietenul (the friend)
plural: prieteni (friends), prietenii (the friends)

Why is it bunicul and not just bunic?