What is the difference between golchi and ymolchi? (removed the 'r from golchi)
The 'r is not part of the verb -- it's the definite article "the", as in "golchi (wash) 'r (the) llawr (floor)".
golchi is "to wash (something else)"
ymolchi is "to wash (oneself)"
Where do you get the word "story" from? The recommended translation for the sentence is "Have you washed the floor?" with "floor".
However, llawr can also mean "story" in the sense that is also spelled "storey" -- a level in a building. For example, a three-storey house (or three-story house) could also be called a house with three floors. Depending on where you are from, you might call them ground floor, first floor, second floor; or first floor, second floor, third floor.
Is 'Have you cleaned the floor' not also a correct translation? The system should cope with some basic synonyms.
Yes. Different words. But different meanings? If so, what is the difference in meaning?
Yes "'r" meaning: the. So, in welsh we have three words meaning the; "'r, yr and y" google: 'r, yr or y and you will find the answer.