two years ago I have worked for a company for some months, I was offered a consecutive job there and I was given a certificate of employment. The certificate was good, but it could have been better from my point of view. I have declined the offer because I wanted to put more work into my academic carreer. I have also asked if it was possible to get some more detailed feedback on my work and what was missing for a better rating in the certificate of employment, but I wasn't given any info other than that I had been performing good but not perfect. Since then I have been offered two jobs by that company. I have declined both because of my studies (and told the recruiters this reason). They have approached me with two more offers in the same year of which I have declined both - again because of my studies. When I declining the last offer I have also written that I won't have enough time to study and work at the same time and that I will approach them when I am done and interested in a position there.
Now, about half a year later, they have approached me again with an offer. I would now have time to work there, but I don't know any exact conditions yet. I'd like to know the detailed conditions and I'd like to ask if (or why not after 4 offered jobs) it wasn't possible to get a change of the certificate of employment. How do I do this without being to rude?
FIY: the job offers I got are for the exact same position that I was working in already for them.
NOTE: A certificate of employment is a letter from your employer in which they rate you and that you can give the recruiters of your future employer when applying for a job. I'd say it's like a letter of recommendation but can more realistically contain negative feedback. In Germany it is usual to hand in certificates of employment from your last jobs when applying for a new job. A missing or negative certificate can have a drastic impact on your chances as an applicant