Why must 'hij' be used and 'he' is incorrect?
'Hij' is always 'hij'. There is no unstressed equivalent like there is with 'jij', 'zij' en 'wij' :)
There is, but you wouldn't use it at the beginning of a sentence and you also don't use it in writing.
"Does he have a dog?" could be translated/pronounced as "Heeft-ie een hond?".
So you change the vowel but still use a long one? Compared to the other unstressed pronouns, this just doesn't sound lazy enough to me! ^^
Why is this een hond and not één hond?
The English sentence reads, "He has a dog..." The Dutch word "een" means a, while "één" means one. So "Hij heeft één hond" would translate to "He has one dog". It basically means the same thing, but the emphasis is different.
...couldn't I use the diminutive here? hondjes/katjes?