bod - to be: preterite tense

Use with 'yn' to form the preterite, which corresponds to English 'have been...' or 'have been to...'. e.g. 

Bues i yn yr Eidal - I have been to Italy
Fues i yn yr Eidal? - Have I been to Italy?
Fues i ddim yn yr Eidal - I haven't been to Italy.

afirmative
bues/bu+m* i 
buest ti
bu(odd) e
bu(odd) hi
buon/buom* ni
buoch chi
buon nhw/buont hwy*

interrogative
fues/fu+m* i?
fuest ti?
fu(odd) e?
fu(odd) hi?
fuon/fuom* ni?
fuoch chi?
fuon nhw?/fuont hwy*

note: in colloquial speech, the interrogative can take two forms: either use the affirmative form and use tone of voice to turn it into a question, or use soft mutation and tone of voice. The soft mutation is because of 'a', the interrogative particle which is usually unheard in speech. 

negative
fues/fu+m* i ddim
fuest ti ddim
fu(odd) e ddim
fu(odd) hi ddim
fuon/fuom* ni ddim
fuoch chi ddim
fuon nhw ddim/fuont hwy ddim*

alternative negative
ni bu+m (but not ni bues)
ni buost
ni bu
ni bu
ni buom
ni buoch
ni buont

note: in some areas you may hear 'ni fu+m', ni fuost, etc. Also note that the final pronoun is often dropped from this form of negative. 

* more formal forms