Both fortement and fort can be used as an adverb

Question

Hi Kwiziqers!  Marc a une forte personnalité. Il défend ___ ses opinions. Why only fortement is correct? Fort can also be used an adverb.

Accepted Answer

Hi Winston,
Thank you for this interesting question and you are quite right that 'fort' and 'fortement' can be used as adverbs to mean strongly. But this lesson is about adverbs that are formed with -ment so you would expect the Kwiz answer to be 'fortement'. In any case, 'fort' would not work in this example.
However, let's look at the different adverbial usages of 'fort'-

Je t'aime très fort,  tu sais = I love you a great deal, you know
Il crie et parle très fort = He shouts and speaks very loudly 
C'est fort probable =  It's highly likely
Ce projet est fort intèressant = This project is most  interesting 
Son cœur bat très fort = His heart beats very rapidly

etc. 
So 'fort' can mean a host of things, meaning with intensity, and this is the kind of words that is given 2 pages for a dictionary entry.
However, 'fortement' is of a higher register and is used in these types of situations :

Il est fortement défendu de stationner ici = It is expressively forbidden to park here
Elle a indiqué fortement qu'elle n'était pas d'accord avec cette opinion = She expressed forcibly that she didn't agree with this opinion
Il est fortement question d'un nouveau confinement à Londres = There is a strong likelihood of another lockdown in London