About a.....

[Sally mills]

In the sentence 'Tu parlais avec ton copain quand le téléphone a sonné'. What is the a for?

[George Hunt]

To form the past tense of a verb  you can use "avoir"  (it is called an auxiliary verb when used this way).

So ,since  avoir is conjugated like this:

j'ai =I have

tu as  =You have

il/elle a  he/she has

nous avons  =we have

vous avez =you have

ils ont= they have

the "a" in "le téléphone a sonné'  corresponds to the use in the 3rd person singular. ("the telephone rang OR has rung")

 

Similarly you might say "j'ai écrit un mot" to say "I left a  (written) message"or

"Ils ont mangé du cheval" to say "they had a nice hamburger"

 

In the example you gave  "parlais" is the past continuous ("you were talking") and so that makes for a little contrast with the (sudden) ring of the phone(if the telephone "sonnait" that would mean it was ringing a few times.)

[Sally mills]

so the ringing of the telephone has to match the parlais?  but if it did not, because I cannot remember that that is what should happen to a telephone, would it be wrong to just say le telephone sonne?

[Charles D]

"I was talking to my friend when the telephone rang."

You describe an event in the past.

Le telephone sonne is perfectly OK, but not in this context.  If you used this construction, you would be saying "I was talking with my friend when the telephone rings."