To distinguish between "I like you" and "I love you", do I say "mwen byen renmen ou" or "mwen renmen ou" respectively? I know that "renmen" means both "to like" and "to love".

You say "renmen" for both.
Of course there are other synonyms to renmen, but there's no need to say "Mwen byen renmen w" unless you want to say "I truly love you".

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