ADVERBS 01 - adding '-ment' - see also ADVERBS 02
A fair few examples of adverbs can be found amongst the words on the "when" and "how" pages in the vocabulary area.
Usually we think of an 'adverb' as something that qualifies a 'verb' - eg something is done 'slowly' ('lentement') or 'rapidly' ('rapidement'). Adjectives in English that have '-ly' added to make an adverb (eg 'slow' + 'ly'), are similar to adjectives in French that have '-ment' added to make an adverb. [But it should be remembered that there are lots of adverbs (in both languages) that aren't created in this way.]
The simple basic rule to remember, for '-ment' endings, is to look at the 'masculine' version of the adjective. If it ends in a vowel (eg 'vrai'), the '-ment' is added to that ('vraiment') - 'really', 'truthfully'. If it ends in a consonant (eg 'complet'), the '-ment' is added to the *feminine* form of the adjective ('complètement') - 'completely'.
And yes, there are other variations on this - primarily adjectives ending in 'ent' or 'ant' - these generally have more than one syllable, and form the adverb differently. The most commonly-used ones are listed below.