Learn English – Asking someone “For how much did you buy it”

When did you born?

is not a grammatical sentence, because born is the past participle of the verb to bear, whereas you must use the bare infinitive after did.

The verb to bear means to generate among other things, so your question can be correcty asked using a passive form,

When were you born?

which is equivalent to, but much more frequent than, *When were you given birth to?"

If your friend wants to answer this question in a grammatically correct way, they should answer I was born on ...

We use "how many" for countables like apples and "how much" for uncountables like sugar or milk. So, of the first two, this one is the better (but still wrong):

How many percent of the work is done?

We use the word percent only if it follows a number- 10%, five percent or whatever. If there is no number, we use the word percentage instead. See percent vs percentage for more information.

We can't ask "how many percentage", because a percentage isn't a number, so instead we ask.

What percentage of the work is done?

See the last example here.