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?

Same definite-article rule:

bunic = grandfather, a grandfather
bunicul = the grandfather / grandpa

In many contexts, bunicul can also be understood as grandpa in the sense of my grandpa / our grandpa, even though the possessive (meu, nostru) is not stated.

So bunicul așteaptă la ușă is naturally understood as:

Grandpa waits at the door, not just a random grandfather.

Does când prietenii sună mean “when the friends call” or “whenever the friends call”?

It can mean both, and context decides.

Romanian present tense plus când can express:

a habitual action: Whenever the friends call, grandpa waits at the door…
a specific time: When the friends call (on this occasion), grandpa waits at the door…

In isolation, most people will read it as habitual, similar to English When(ever) the friends call…

Why is the verb sună singular if prietenii is plural?

In Romanian, the 3rd person plural and the 3rd person singular of some verbs look the same in the present tense. A suna is one of them:

eu sun
tu suni
el / ea sună
noi sunăm
voi sunați
ei / ele sună

So prietenii sună is 3rd person plural, even though it looks like the 3rd person singular form. The subject prietenii (the friends) tells you it’s plural.

What does la ușă literally mean, and is it the same as “at the door”?

la ușă literally is at the door / by the door.

la can mean at, to, by depending on context.
ușă = door (indefinite)
la ușă = at the door

You could also say:

în fața ușii = in front of the door (a bit more specific about position)

In everyday speech, la ușă is the natural equivalent of English at the door here.

Why is it le spune salut and not le spune salutul?

Here, salut works like an interjection or a fixed greeting:

salut = hi / hello

In Romanian, when you report a greeting with verbs like a spune (to say), a zice (to say), a striga (to shout), the greeting is usually left without an article:

le spune salut = he says hello to them
spune bună ziua = he/she says good afternoon

salutul would be “the greeting / the salute”, more like a noun referring to a specific greeting as an object, not what you normally use as a spoken hello in this structure.

What exactly does le mean in le spune salut?

le is a clitic pronoun, 3rd person plural indirect object (dative case), for both masculine and feminine:

le = to them

The verb a spune (to say / to tell) typically takes an indirect object:

spune ceva cuiva = say/tell something to someone

So:

bunicul le spune salut
= literally: grandpa to-them says hello
= grandpa says hello to them (to the friends).

The direct object is salut (what he says); le marks the people receiving the greeting.

Why le and not îi in this sentence?

Because in this sentence, the pronoun is indirect object, not direct object.

For 3rd person plural:

direct object (accusative, people): îi (masc.), le (fem.)
indirect object (dative): le (for both genders)

a spune needs an indirect object (to whom?), so you must use the dative form:

correct: bunicul le spune salut = grandpa says hello to them
using îi here would sound wrong to a native speaker in standard Romanian.

Can I also say spune-le salut instead of le spune salut?

Yes, but the meaning and tone are different.

le spune salut – normal declarative sentence:
He (grandpa) says hello to them.
spune-le salut – imperative:
Say hello to them. (you are giving an order to someone)

In declarative sentences, clitic pronouns like le normally go before the verb: le spune.
After the verb (attached with a hyphen) is typical of the imperative: spune-le!

Could I change the word order to Bunicul așteaptă la ușă când prietenii sună?

Yes, that is perfectly correct Romanian.

Both are fine:

Când prietenii sună, bunicul așteaptă la ușă și le spune salut.
Bunicul așteaptă la ușă și le spune salut când prietenii sună.

The difference is just emphasis and style:

Starting with Când prietenii sună slightly emphasizes the condition/time.
Starting with Bunicul puts the focus more on grandpa’s action.