Using de laquelle vs à laquelle

Question

Could someone please clarify the rule for choosing between à laquelle and de laquelle in a sentence such as:

"The girl I'm thinking of is Isabelle." : La fille ________ je pense est Isabelle. "

I understand that they both translate "about/of whom" but they are not interchangeable.

Thanks in advance!

Kalpana

Accepted Answer

Bonjour Kalpana,
Chris's answer is great! To supplement, the preposition "de" or "à" depend on the verb you are going to use. 
Penser à [quelqu'un] = to think about [someone]

Je pense à la fille ->  la fille à laquelle je pense
I am thinking about my friends -> the girl whom I am thinking about

Parler de [quelqu'un] = To talk about [someone]

Tu parles de la fille -> La fille de laquelle tu parles
You talk about the girl -> The girl whom you are talking about

See links here: Verbs + à + [indirect object]  -  Verbs + de + [indirect object]  -  Verbs followed by à and de
I hope this is helpful.
Bonne journée !

Answer

you use de laquelle whenever you'd use the preposition de and à laquelle goes with the preposition à. The choice of preposition is determined by the verb.Je pense à la fille. --> La fille à laquelle je pense. -- The girl of whom I'm thinking.Tu parles de la fille. --> La fille de laquelle tu parles. -- The girl about whom you're speaking.

Answer

With super great respect, Céline - your second orange line example above is headed Parler à, but the example itself  has Parler de.

Answer

Bonjour Anne,You are correct! Thank you for letting me know! ;-) I've amended this example.Merci et bonne journée !