“I won’t be able to use much the computer today. I have to go to a place.”
“It’s incredible how flies the time! It will be Christmas soon!”
Sorry, I am still learning English. I hear many people who have second-language English that is my friends say that though.
“I won’t be able to use many computers today.” See how much/many in this position would indicate a multiple of computer? As though you could only use a few. But you’re not wanting to count the number of computers but the amount of time. In the original, “the” before “computer” indicates it’s singular so that the original sentence is simply not grammatical. What you probably mean is: “I won’t be able to use the computer much today.”
(Edit: I think in older English you could also put “much” earlier in the sentence: “I won’t much be able to use the computer today.” Makes me think of “Tell me where is gandalf, for I much desire to speak with him.” https://youtu.be/uE-1RPDqJAY?t=74)
For the second sentence, swap the verb and the noun. I don’t know why. “It’s incredible how the time flies!” Putting the verb before the time might fit in a question: “How flies the time? Like an arrow!”