[Originally posted on 05/06/12 on the Duolingo French for English speakers forum by ricainrico]
To form most adverbs, we generally take the feminine form of the base adjective and add “-ment”.
heureuse – heureusement, claire – clairement etc.
However, if the masculin form of the base adjective ends in “-ent” or “-ant”, we have to double up the “m” for the adverb ending “-mment”.
récent – récemment, suffisant – suffisamment etc.
The double “m” doesn’t change the pronunciation at all, it’s just one of those lovely spelling exceptions you have to deal with!
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