i had to find someone who had had the same level of experience as i had.

How would the previous sentence be correctly capitalized?
Answer: I had to find someone who had had the same level of experience as I had.
if your pc opens the a65 file, but it's the wrong application, you'll need to change your windows registry file association settings. in other words, windows is associating a65 file extensions with the wrong software program.

How would the previous sentence be correctly capitalized?
Answer: If your PC opens the A65 file, but it's the wrong application, you'll need to change your Windows registry file association settings. In other words, Windows is associating A65 file extensions with the wrong software program.
it is a generic term for suddenly occurring interrupted blood supply to the brain resulting from a blood clot occurring in the brain, a blood clot breaking free from somewhere in the body and getting stuck in the brain, or a cerebral haemorrhage in which a cerebral artery bursts.

How would the previous sentence be correctly capitalized?
Answer: It is a generic term for suddenly occurring interrupted blood supply to the brain resulting from a blood clot occurring in the brain, a blood clot breaking free from somewhere in the body and getting stuck in the brain, or a cerebral haemorrhage in which a cerebral artery bursts.
any discussion about a low-carbon future that relies on a significant share of renewable energy must necessarily focus on the idea of “smart grids.”

How would the previous sentence be correctly capitalized?
Answer: Any discussion about a low-carbon future that relies on a significant share of renewable energy must necessarily focus on the idea of “smart grids.”