Difference between "This is" and "It is", "These are" and "They are"

Question

When I should use "It is" and when "This is"?

For example when I show an apple to my son, how is better to say:

1. It is an apple
2. This is an apple

What is the main difference between abovementioned phrases?

The same problem when I show scissors, glasses etc. (plural nouns)

1. These are glasses
2. They are glasses - sounds very awkward to me.

Answer 1

You say “This is an apple.” while gestering with the hand to indicate what this refers to. Using it means you have already established a subject and can repeat it. E.g. you might continue with “It is good for you.”. Don’t use it without an antecedent. Do use this when presenting the thing being referred to.

For the second, when you first start to talk you hold the glasses in your hand, point to them,or somesuch, and say “These are glasses.” Now that you have established a subject, you can continue with “They must be kept clean.” If the idea of glasses being plural in English is just odd sounding, ponder the fact that it’s a pair of glasses. To refer to one lens only, or any other component like a screw at the temple, would need explicit focus on that part. “These are my glasses. One lens corrects astigmatism and the other is plain glass.” So talking about the set, “I wear a pair of glasses. They correct my vision.” That’s the same as “I wear glasses. They correct my vision.” Think about the pair to justify why it sounds that way.