The difference between formal English and informal English

[Tdol]

Not necessarily. If you are writing a text where you can use contractions, it doesn't mean that you have to. There are good reasons for varying forms- emphasis, etc.

[MikeNewYork]

Thanks for your replies. :)

If I write text and use "it's", then would it be inconsistent or wrong to use "they are" in same text ?

That depends, as TDOL said. I would have to have a reason however, to put both forms in the same sentence.

[Penguin_vic]

Can you explain to me what's the similarities between 2 of them? :( I'm having an essay on this :( Thanx a lot ^^