[Negative of] "I had to buy a pen"

[Heisenberg]

Hey,

I wanted to convert this into a negative sentence and I got confused somehow. What will be the correct negative structure of this sentence?

I did not have to buy a pen.
[or]
I had not to buy a pen. (someone told me that this is the right answer, but I don't think it would be)

Please also explain how you arrived at your specific answer. It will help me understand the process.

Thanks.

[Alan]

The simple denial of have to (do) is not have to do and so

I didn't have to buy a pen.

means simply "it was not necessary for me to buy a pen".

Have not to (do), on the other hand, is not simply a denial but the contrary of the original statement, i.e. a prohibition. Thus

I had not (hadn't) to buy a pen.

means "I was not allowed/permitted to buy a pen".

This latter construction is, however, primarily British English (and a little dated at that), and an AmE speaker would be more likely to say "I wasn't allowed..."