I will talk about This, That, and It. They are all pronouns.
(1) "This" and "That" are usually taught together.
This refers to someone or something that is close to you in location. By close, let's say "within arm's reach" or close enough to touch with your finger. So you can say,
This is my wife.
This is my cat.
This is the worst building.
This is beautiful.
when your wife, cat, the worst building, and the beautiful object are close to you (for instance, you can touch them with your finger). You can also say "This is" when you are looking at a photo of your wife, your cat, etc. For instance, "This (person in the photo) is my wife."
By contrast, you use that when you refer to something that is not located close to you. For instance, if your wife or cat is across the room (too far away to touch them), you can say "That is my wife, that is my cat." And if the worst building is also not very close to you, you can call it "That worst building."
So, This refers to something or someone close to you in distance, That refers to something farther away (not close to you). There is no clear cut rule as to "how far" away something must be to decide which word to use. (In practice, it is a matter of the perception of the speaker. And I suspect you have similar words in your native language.) To oversimplifly, if something is close enough to touch, use "this." Otherwise, use "that."
(2) It
It is a pronoun. It is the third person, singular, neuter pronoun. This means it usually refers to objects, not to people, animals, or other living things.
So, most the time, we would not say:
*It is my wife.
*It is my cat.
Instead, we would say:
She is my wife.
She (or he) is my cat.
Your other two examples are correct:
It is the worst building.
It is beautiful. (If the "it" refers to an inanimate object)
But what if "it" refers to your wife or cat? Hopefully, you have learned by now that you should say
She is beautiful. (Or: He is beautiful, if it is a male cat.)
I should stop now, but for some completeness, there are a few times you can say
It is my wife.
It is my cat.
I will give you just one example:
If you are showing a photo of your wife to your friend, and your friend asks you who is in the photo, you can answer "It is my wife."
They're both correct, but they mean slightly different things.
In "I could take care of them myself", the myself emphasizes that I'd be the one taking care of them, as opposed to someone else. It doesn't really change the meaning of the sentence, it just emphasizes a certain aspect of it.
In "I could take care of them by myself", the by myself means "without help from others".
Best Answer
It is so-called "dummy" or "expletive" it. It acts a placeholder, as a proxy subject. This structure using it divides a simple subject-verb-complement utterance
into two pieces, in order to put some emphasis on the first piece:
The speaker is making a suggestion, although in a somewhat circuitous manner. Paraphrase: You should come to my office...
If you wanted to substitute that for it, you would have to change the order of the clauses, since that as demonstrative pronoun refers to something already mentioned or present:
Dummy / placeholder it as proxy subject can anticipate the actual semantic subject. Demonstrative pronoun that must refer back to one.
These are colloquial statements which strike a casual tone.