When I hover over this word, it comes up with "To laugh, laugh, laughing" but when I enter "laugh" it marks it as wrong. Is there a slight change to the word that I'm missing?
On its own it is best translated as 'to laugh' or 'laughing' When used in combination with, say dw i, dych chi, etc it can end up in English as 'laugh' or 'laughing':
- Chwerthin - to laugh, laughing
- Dw i'n chwerthin - I am laughing, I laugh (but not '*I am laugh')
- Dw i'n hoffi chwerthin - I like to laugh, I like laughing (but not '*I like laugh')
If you look up 'laugh' in a dictionary you will get something like 'chwerthin be', where be stands for berfenw (verb-noun). You will also find, probably, 'chwarddiad eg, chwerthiniad eg' where eg stands for enw gwrywaidd (masculine noun)
Duo does not work like a dictionary, though, so where the words appear on their own we generally use a convention of 'to laugh' and 'laughing' for the verb-noun, and 'a laugh' and 'laugh' for the noun.