Ron doesn't know how to speak parseltongue. He has merely heard Harry speak the word "open" in parseltongue twice (once in book 2 and once in book 7). He simply imitated the sound over and over until he made a sound close enough to the actual pronunciation that the spell reacted.
Note that when Ron hissed at Harry that Harry didn't understand the hiss. That is because Ron wasn't speaking the language. He was just parroting a sound.
I recall a Potter fanfic in which the password to Snape's Hogwarts quarters is in parseltongue and cannot be changed because nobody speaks the language and thus cannot change the password. All they can do is memorize the specific hissing sound that opens the door.
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"Anyone can be a winner if their definition of victory is flexible enough." - The DM of the Rings XXXV
Note that when Ron hissed at Harry that Harry didn't understand the hiss. That is because Ron wasn't speaking the language. He was just parroting a sound.
I recall a Potter fanfic in which the password to Snape's Hogwarts quarters is in parseltongue and cannot be changed because nobody speaks the language and thus cannot change the password. All they can do is memorize the specific hissing sound that opens the door.
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"Anyone can be a winner if their definition of victory is flexible enough." - The DM of the Rings XXXV