Translation:I would not run if it were raining.
Fyddwn i ddim yn rhedeg? Why not Faswn i ddim yn rhedeg? Fyddwn is the plural future, faswn i is conditional.
You may be mixing up two different persons in two different tenses:
- bydda i - I will be (1st person singular future of bod)
- byddwn ni - we will be (1st person plural future)
- byddwn/baswn i - I would be (1st person singular conditional)
- bydden/basen ni - we would be (1st person plural conditional)
As EllisV says, the use of the if-conditional tasai in one part if the sentence usually indicates that there will be a conditional in the other clause.
Diolch! Thanks for having patience with me! I am 67 years old and I am not so good at english.