Qui is only seldom substituted by lequel, laquelle, lesquels, lesquelles ersetzt

If you want to keep it simple then go with the rule: lequel, laquelle, lesquels, lesquelles cannot be subject of the subordinate clause.
However, things are a bit more complicated, you can have it. In French (as actually also in English) we can separate the restrictive and non-restrictive relative clauses. What now is a restrictive relative clause? It is a relative clause that defines the referred object (also defining relative clause).

The man who has a black beard and a big hat is my father.
The table that stands over there in the corner is a nice dining table.

Without the definition we would not know who or what actually is meant.

In opposition to restrictiver there are non-restrictive (or also descriptive) relative clauses, which give only additional information about an already defined thing or person. The sentence would be perfectly understandable also without this additional information.

My father, who wears pink shorts and a big hat in this photo, has an aweful taste in fashion.
The author of the latest bestseller novel, who is gaining millions and millions nowadays, used to be poor.
The little round table in the corner, which I like very much, is one I got from my grandparents.

Now, coming back to French relative clauses and substitution of qui:
Lequel can only substitute qui as a subject in a non-restriktive (descriptive) relative clause.
In restrictive relative clauses, qui cannot be substituted.

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