How to use “by” and “until” 

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I've watched 10 movies until now. He should have been here by now I think in the first sentence, "by" doesn't work and using it will not be idiomatic. This applies to the using "until" in later sentence too. For me using "until" will not be be a correct usage. Instead the "until" in the first sentence and "by" in the second sentence fit the best. Am I right?

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The difference is that 'by' refers to an event that happens at a moment in time, whereas 'until' refers to an event that happens for a period of time. They can't usually be used as alternatives of each other - except perhaps when used with negation. Using your second example, to say "He should have been here by now" means that at some point in time, prior to this moment, he should have come here. To say "He should have been here until now" would mean that from some earlier time, continuing to this moment, the person should have stayed here. To reply to your 3rd comment to whatever's answer, no, you cannot substitute by for until in all those examples. As mentioned, it doesn't mean the same thing, but in those cases it makes nonsensical sentences. In the examples of: She won’t be back (by) 5 o’clock. - She won’t be back (until) five o’clock, they almost mean the same and can be substituted. This is the negation I was referring to. Technically, "She won’t be back (until) five o’clock" could mean that she will turn up at five o'clock on the dot, whereas "She won’t be back (by) 5 o’clock" means that time she will come back will be after 5 o'clock