I had an interview with a company but didn't pass. However, they offered to give me another shot after 6 months (due now). How to write an email to enquire about the interview in a polite manner?
Contact their HR, if possible the same person you were dealing with 6 months ago. If they seriously wanted you to reapply then they should have left you contact details of some sort. However don't get your hopes up too much, sometimes they would say this without having any real intention of giving you another chance.
However it's also possible that you didn't quite fit their experience requirements the first time, so they do want you to reapply.
Either way, contact their HR to find out. Probably an initial phone call would be the fastest way, then if they don't have any positions open, you don't waste much time and effort.
It would be perfectly ok to reach out directly to the person that previously conducted the interview, assuming you have the contact information. I would address the person directly and let him or her know that you are following up based on the prior agreement.
If, during the 1st interview, the interviewer shared info about interesting work or project the group was working on, I would add a sentence to check on project status, and re-emphasize how much this type of work appeals to you and fits your skills and strengths.
Given that 6 months have passed and you don't want people to think that you have been sitting around with nothing to do, I would also touch on what you have been doing in the meantime to advance your skills, hopefully putting you in a better position to land the job.
As you have their email address, dropping them a note should be absolutely fine.
I would adopt a warm tone, with evident enthusiasm for the company. Be sure to let them know what you've done in the intervening period to further develop as an interesting prospective employee.