'This' is more specific than 'it'. 'This' is demonstrative, so it generally refers to something very close. For example, if I were showing you my cat, I would say, "This is my cat." 'It' is a pronoun, so it takes the place of a noun. It is more general. A few examples: "It is raining." "This is a difficult task." "It makes me happy." "This evening, I'm going out to dinner." Sometimes you can use either one. For instance, I could either say "This is your decision" or "It is your decision". I hope that helps!

"This is" usually refers back to something you have said or just written to define "this" in the context of the statement you are making. "I heard his dad had recently died. That is why he needed to leave work early." Whereas I might say or write "It is" and the it will be defined in the sentence I am speaking or writing at that time. "It is sad that his dad recently died." So generally "this is" demands that you think back over what you just read and "it is" asks you to wait for the definition to be given. Hope that helps clear things up for you!

This is indicates something you have near or in your possession. It is, or that is, indicates something not near you. "This is mine and you can not have it." "That is mine, please give it back." For the most part, as far as I know, 'that is' and 'it is' are interchangeable.

I thank everyone of you! Each answer was different, but I could understand