Use of "dont"

Bryce, 

because in this context,  it is  ' penser à qqn ', not ' penser de ...'.  - Je pense à Isabelle.

As a relative pronoun, dont can only 'replace' the complement when it would be introduced by the preposition ' de '. 

When the preposition introducing the complement being replaced would be " à ", you need to use ' à qui ' or ' auquel (or its variants), as the lesson is covering. 

( With any preposition other than de, the basic form is ' preposition qui/lequel/….. ', with the usual contractions when the preposition is ' à '. ) 

See ' à noter ' in the Lawless link on 'dont' below.

 https://www.lawlessfrench.com/grammar/penser-lesson/

 https://www.lawlessfrench.com/grammar/relative-pronoun-dont/

 À + qui, auquel, à laquelle = to whom, what, which (French Relative Pronouns)

This puzzled me too but apparently it's just because dont replaces a phrase with de, not a phrase with à.

I agree with Bryce.  I can make no sense of this lesson.  The sentence clearly uses "of".  Or, using more correct english, it would be: The girl of whom I'm thinking.  It's not The girl to whom I'm thinking.

I agree with Bryce.  I can make no sense of this lesson.  The sentence clearly uses "of".  Or, using more correct english, it would be: The girl of whom I'm thinking.  It's not The girl to whom I'm thinking.

Mike,

Yes, this is ‘ of whom I am thinking ‘, and that it is why the translated verb form in French is  ‘ penser à (Isabelle) ‘.  

You cannot try to understand this in French just by looking at the preposition in isolation. Prepositions, in both English and French, often have more than one meaning and are used idiomatically in many contexts.

The speaker is ‘ picturing (imagining) ‘ Isabelle as they speak, not ‘ expressing an opinion ‘ about her.

There is a small group of the verbs that require a following preposition that can be followed by either the preposition à or de - eight common verbs have a change in meaning depending on which preposition is used. 

‘ Penser à ‘ and ‘ penser de ‘ are examples of this. 

It is a difficult example because of the verb used, but this is a lesson at B2 level, and the difference between using ‘ auquel ‘ and ‘ dont ‘ as the relative pronouns here is something a native speaker knows ‘ inherently ‘.

See the link below : 

https://www.lawlessfrench.com/grammar/a-vs-de-2/

Thanks Maarten!  Your explanation and the lesson, which I hadn't seen, make things much easier.  I have long been frustrated by the seemingly random use of "a" and "de" after verbs.  Now it makes sense.

Thanks Maarten!  Your explanation and the lesson, which I hadn't seen, make things much easier.  I have long been frustrated by the seemingly random use of "a" and "de" after verbs.  Now it makes sense.