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Considering following sentences.

  • Not sure if I look so retarded or people don't understand me.
  • Not sure if I walk so fast or others walk too slow.
  • Not sure if I'm doing something wrong or you are trying to fool me.

Questions:
Is it formal? if no how does formal one look like?
Is it possible to use "that" instead of "if"? the difference?
When should I use this form? and is there any other way to say it?

Thanks in advance :)

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3 Answers 3

up vote 4 down vote accepted

1. Is it formal?

It doesn't sound especially formal to me.

2. if no how does formal one look like?

I would use whether to make it sound formal:

  • Not sure whether I look so retarded or people don't understand me.
  • Not sure whether I walk so fast or others walk too slow.
  • Not sure whether I'm doing something wrong or you are trying to fool me.

3. Is it possible to use "that" instead of "if"?

As StoneyB points out that doesn't work because of or.

4. the difference? When should I use this form? and is there any other way to say it?

Use if or whether if you want to present a list of possibilities:

  • I'm not sure if/whether A, B or C

Use that if you want to present only one possibility:

  • Not sure that I look so retarded.
  • Not sure that I walk so fast.
  • Not sure that I'm doing something wrong.
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Many thanks buddy, thanks. –  user2889419 Feb 27 at 13:33

The word 'that' probably does not suit there. What about thinking of some better option, the word whether? The reason is, when we have two alternatives, a sentence is better described with whether.

In your examples, neither of those two parts is conditional. For instance, your fast walking does not depend on the other clause and vice versa.

We use if if the sentence is conditional.

More information here.

Broadly, that describes, if puts a condition, and whether gives options.

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In this case, both if and whether can be used with the same meaning. Whether isn't any better, only slightly more formal. –  snailboat Feb 28 at 7:48

There’s no a ‘conditional if’ (it does not introduce a subordinate conditional clause) in your examples, so you may replace it with ‘that’ or ‘whether’.
Vice versa is not possible, you cannot use ‘whether’ when there is a conditional clause.

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1  
That does not work here, with or. –  StoneyB Feb 27 at 11:57
    
You are right,thank you, that can introduce a condition: 'Now that I know I can do it'. –  Lucian Sava Feb 27 at 12:08

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