This / That; These / Those

[jack]

When do I use 'this' and 'that'? What do these mean?
1. This is a car.
2. That is a car.

When do I use 'these' and 'those'? What do these mean?
3. Do these programs run very slow on your computer?
4. Do those programs run very slow on your computer?

Scenario:
I saw a car driving on the street last night.

To continue on with my story, do I say:
5. This was the car that I saw.
6. That was the car that I saw.
How do I know which one to use? Do they mean the same thing?

I don't get this, how do I use 'the' and 'a' ? What's the point of using 'the' vs 'a' here? Let's say I was telling someone a story, how do I know if I need to use 'a' or 'the'? If I use 'the', how do they know which street I am talking about?
7. I saw a car driving on the street last night. (What does this mean when I use 'the'? If I'm telling a story how does he know which streeet I'm talking about if I use 'the'?)
8. I saw a car driving on a street last night. (What does this mean when I use 'a'?)

Is this correct? Can I use two 'ifs' here?
9. If I'm telling a story how does he know which streeet I'm talking about if I use 'the'?

Are these correct?
10. Is this correct? Can I use two 'ifs' here?
11. Is this correct? Can I use two 'if' here? (With that context, 'if' is countable right? So I should use 'ifs' not 'if'?)

[blacknomi]

1,5- "This" refers to a thing that is closer to you.

2,6- "That" refers to a thing that is farther to you.

3 and 4 need more context. I installled some programs a couple of minutes ago, and these programs slowed down my PC. Weeks later, those programs still bite. In this case, we take into "time and space" factors. In your example, I'd think that "Do these programs run very slow on your computer?" means the programs you are running or that are already installed, while "Do those programs run very slow on your computer?" means other programs.

5- "This" sounds more emphatic.
7 and 8, I think they are the same, meaning on a certain street.

9- Ok for me but you need a comma after story.

I'd say two "if", need teachers' confirm.

[jack]

Thanks.

[Casiopea]

Try,

Can I use two if's here?
Can I use If ... if here?

There's no need to use quotation marks ('..') . Just highlight the word, then click on the symbol I above. It stands for Italics.

[blacknomi]

You're welocme and thanks, too.

[jack]

Thanks.

Can I use two if's here?

1. Can I use two ifs here? (Is it better to use an apostrophe for 'if'? So there will be less confusion? Does it matter if it is ifsor if's?)

Can I use If ... if here?

2. I don't really get this. How does it work?

[Casiopea]

Originally Posted by jack

Thanks.
1. Can I use two ifs here? (Is it better to use an apostrophe for 'if'? So there will be less confusion? Does it matter if it is ifsor if's?)

2. I don't really get this [If ... if]. How does it work?

You're welcome.

In answer to question 1., with lower case letters, it's best to use an apostrophe. Otherwise, ifs could be interpreted as a typo. if's is best because it's clear. With upper case letter, or capitals, an apostrophe is not required.

In answer to question 2., which refers to sentence 9. below, If ... if represents the words, or structure you want to emphasize. The dots (...) represent all the words between the first If and the second if:

9. If I'm telling a story how does he know which street I'm talking about if I use 'the'?

In other words, Can I use If ... if? is another way of asking, Can I use two if's in the same sentence?

[jack]

Thanks.