haven't and don't have

AnonymousI haven't a pen. I don't have a pen.

Both are correct. The second is more common nowadays.

For me, "I haven't a pen" feels pretty unnatural in ordinary conversational English. In the UK, the most common way to say this in ordinary conversation is "I haven't got a pen". "I don't have a pen" is also fine; it feels slightly more formal.

Please note that the word "I" should always be capitalised.

I haven't a pen wouldn't be very unusual if followed by a defining relative clause, eg. I haven't a pen which writes on glass.