Using possessive - formal to slang


After the 3 lessons on the subject in Level 2 Old Course, I’m starting to get more comfortable with possessive adjectives and their mutations. That is, I’m comfortable with the mechanics, but I’ve started to wonder about when you’d use each version – i.e. double possessive, or a single possessive before or after the (verb)noun.

The tutors in the challenges use them all but I think this is to get you used to hearing the different versions, rather than a guide as to when to use each.

So, is there a difference between how ei phenblwydd hi, ei phenblwydd and penblwydd hi come across to native speakers? Does the double possessive sound a bit formal or stilted in casual speech, and ‘penblwydd hi’ a little too slangy? (Just as in English you wouldn’t say ‘I dunno’ in a job interview…)

Or are the differences too slight to worry about?

(I’m ignoring emphasis here, where I assume you’d use ‘ei phenblwydd hi’ to make sure you know exactly whose birthday you’ve missed…)