I’m guessing that the use of fod o’n or bod hi’n depends on gender. If this the case, is there any way of knowing if an object is masculine or feminine & how much does it matter if you get the gender wrong?
yes, it’s because Welsh nouns are assigned a gender. There are a few that can be both masculine and feminine - often an ‘area’ thing (e.g. N and S) - but really there is no use looking for ‘rules’ to know which are which because there are so many exceptions, the rules often don’t apply!
Once you hear things a lot (from 1st language speakers, TV or radio), certain ones will start to stick because they sound ‘wrong’ otherwise, but to be honest even the 1st language speakers I work with very often ask whether things are M or F!
Of course it’s best to try and get them right, but even if you do get a gender wrong (which I do even after over 20 years of speaking Welsh every day ), most Welsh speakers aren’t pedants and will just be chuffed that you’re learning the language!