Perhaps I wasn’t very clear in the title.
What I’m searching for is a way to map the f key to execute some command, but only if I’m not in the middle of another series of a key strokes like cf<char> or df<char>.

Is there a way to know that?

Thanks a lot!

(I’ve searched this site and didn’t find similar question).

>Solution :

In cf<char> and other operator+motion scenarios, Vim enters a special mode after the operator, "operator-pending mode", in which mappings created for other modes don’t apply.

Therefore, you could map f in normal mode without impacting its behavior in operator-pending mode:

nnoremap f <Cmd>echo 'foo'<CR>

That said, f is a useful command in its own right so it might not be a good idea to override it with something unrelated. Also, f in normal mode and f in operator-pending mode have the same semantics. By changing the meaning of one f, you make the other f less useful.