Bear - you've asked two very good questions. Here's my simple answer: 1 use "the" because it is identifying something that includes all "computers in the world". It is describing an invention, the type of invention/the thing that was invented... the computer, the car, the telephone, the x-ray, etc. 2 you were ten years old in the past / you found/discovered/realised/believed... in the past, so what you found "was" a powerful tool. Everything in this sentence is talking about or referring to something in the past. "I was told that the sun rises in [not from] THE east" uses the present tense because it rises every day, (1) it is a repeating or regular action (an action that happens all the time and is expected to happen again in the future), and (2) an situation that is permanent. This is one the uses of the present tense verb (not just for something is happening now). (1) For example: I exercise every day. I always try my best. I usually sleep at night. (2) I live in Canada. Water freezes at 0 degrees C. I hope (at this moment) (not an example of (1) or (2)) that my answer has helped.

1) Yes, use "the". "I found that computer..." makes no sense whatsoever. 2) This is a tricky question, and many students go wrong because they assume they can use the "universal truth" rule every time. Most of the time, we use past tense. Anyway... - if what you discovered in the past somehow relates to the current situation, then use present tense. - if what you discovered in the past relates to your experience in the past, then use past tense. It really comes down to what you are telling your reader.

-when I was 10 years old, I found that computers were a powerful tool. -when I was 10 years old, I found a computer was a powerful tool.

1) It depends on the context you use. For example, you could say "the computer I saw yesterday" (the specific computer) or "I need a computer to check my email" (a = any computer, one computer) or "I like my computer" (no article since you use a possession word) or "Computer is the great invention of humankind" (computer in general)