"Where are you going?"

Translation: Ble rwyt ti'n mynd?

[CivisRomanus]

Up to this level, the interrogative form of 'bod' for the 2nd singular person has always been 'wyt' (e.g. wyt ti'n mynd i'r parc? ), while 'rwyt' has been used for affirmative sentences (rwyt ti eisiau coffi ).

In this sentence, is 'rwyt' used because an interrogative pronoun (or some other word) comes before it (in other words, because the sentence does not start with 'wyt')? For instance:

pam rwyt ti'n mynd i'r parc?  as opposed to  wyt ti'n mynd i'r parc?

or 

am faint o'r gloch rwyt ti'n chwarae sboncen?  as opposed to  wyt ti'n chwarae sboncen?

Or is there some other reason for using 'rwyt' in an interrogative sentence?

Thanks in advance.

[Beth743129]

I tried "Ble sy dych chi'n mynd?" and got it wrong. Would you please point me to the discussion where "sy" is explained? Many thanks.