My Question is How to Remove Dark Yellow Lines Occurred on teeth due to Hard illness when I was 7 years old. (photo)

To achieve the very best smile, you should consider teeth straightening and whitening first.  You will then need porcelain veeners to complete your smile transformation.  Straightening and bleaching first allow for the the least drilling and most natural final smile

Great question--you actually have both light and dark color changes to your teeth--If cost is not a concern porcelain veneers would give you the best long term improvement. A more conservative approach is to bleach the teeth first then to match the defects using opaquers and composite--a trained cosmetic dentist can do a great job for you either way--You can definitely have an awesome smile. Good luck

I have had some success performing a procedure called Enamel Micro-Abrasion. Sometimes these stains extend only slightly below the surface.  Ask an experienced dentist if they think they could use a fine diamond burr to explore this option. You may get a pleasant surprise.  If it extends deeper,  composite bonding could be placed over the deeper prepared areas and finished off evenly with the surface of your enamel. Another option would be to place porcelain veneers over these teeth. I would recommend getting an orthodontic consultation before doing any extensive work. Douglas Jopling,  DDS Dallas,  Texas area

Thanks for posting the picture. Unfortunately the only way to improve the appearance of your teeth will be to place veneers over the stains. There are no whitening methods that will improve the discoloration so. Ali would suggest doing your front 4 teeth to give a more harmonious appearance. Best of luck.