How do you choose between stressed 'to be' and unstressed?

I am wondering in which context people stress the "be" in English grammar.

Both Wiktionary and Wordrefence have a stressed and an unstressed form in their dictionary.

I know that, for example, the verb "to have" is used in strong form only when used negatively:

"I have seen her." /aɪ həv si:n hə/

and

"I haven't seen her." /aɪ hævnt si:n hə/

But for the verb "to be", I can't find an example where we could use it negatively.

Your rule for when to use strong or weak form for has is incomplete. You also use strong form when the word is emphasized:

He doesn't drive his car to work, but he has one. (strong form)
I have two, but he only has one. (weak form)

or at the end of a phrase:

it's the only kind he has.

Both of these cases (and probably others that I've forgotten) are also possible with be.