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I am always confused with these three words when speaking and writing . it , that , this.

For example, (speaking )

A : I swim every day

B: It's good / that's good.

A: I can help you with your homework

B: I really appreciate it / that,/ this

(writing )

The governement is laid back and doing nothing about it. That / it/ this makes things worse.

Could you explain when it / that / this is used in an appropriate usage ?

One more.

For me / to me .

For example,

That's good for you

That's good to you.

That method was very useful for me.

That method was very useful to me.

They are annoying to me

They are annoying for me.

Is there a specific rule ???
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See It vs that.

CJ
Comments  
These are both good questions.

OK, first of all:

Use "it" when there is an antecedent that could be feasibly inserted in its stead.

For instance, you could say "give the dog its bone" because you could insert "the dog" for "it":

Give the dog (the dog's) bone.

You could also say "give the dog this bone", but you would have to be holding it. "This" implies spatial, temporal, or referential nearness.

You could even say "give the dog that bone", but this implies that you most certainly aren't holding it. "That" implies spatial, temporal, or referential distance.

To take your examples one-by-one:

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A : I swim every day

B: It's good / that's good. <---You would say "that's good". It wasn't you who swims. There is distance. There is no antecedent for the pronoun, "it".

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A: I can help you with your homework

B: I really appreciate it / that,/ this

I'm sad you thought of this one. This one is a special case. Notice that there is no antecedent for "it", and there is distance (it is not you who will be helping). The proper way to say this is "I really appreciate that."

BUT

This is a spoken-English colloquialism. A native-English speaker would say "I really appreciate it." The implied antecedent here is "your help." It is technically incorrect, but this is how everyone talks. Confusing, I know. Sorry.

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The governement is laid back and doing nothing about it. That / it/ this makes things worse.

Here you would use "this." This is because it is an idea which you have just said. You are close to it, referentially speaking.

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As for the to/for question:

Use "to" when the qord it is modifying is an indirect object of a verb.

For example:

Give me that. <---The indirect object here is "me", so this sentence could be phrased "Give that to me."

Use for when you are answering the question: "why?"

I bought a gallon of paint. <----Why did you buy a gallon of paint?

I bought a gallon of paint for my bedroom walls.

The word "for" must always be followed by a noun. "To" can be followed by a verb.

For example:

I bought vegetables to eat. <---"eat" is a verb; it can follow "to".

I bought vegetables for eat. <---"eat" is a verb; it cannot follow "for". It must be reworded:

I bought vegetables for eating. <---here "eating" is a gerund, a noun. It can follow "for".
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I hope this helps a little.
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 CalifJim's reply was promoted to an answer.
mulderFor example, (speaking )

A : I swim every day

B: It's good / that's good.

A: I can help you with your homework

B: I really appreciate it / that,/ this

(writing )

The governement is laid back and doing nothing about it. That / it/ this makes things worse.
mulderThat's good for you

That's good to you.

That method was very useful for me.

That method was very useful to me.

They are annoying to me

They are annoying for me.
for can connote benefit: good for you, useful for you -- you benefit in some way. annoying for me doesn't quite fit this idea, so it would be unusual to say that.
to is more neutral.

CJ
I really appreciate it/that/this.

I use it when referring to future tense. I use "that" for past tense. I use "this" for present tense, and starting now or have recently started.

I hope that helps.
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