OT: Phonetics (IPA)

I have a few questions relating to some sounds in English.

1. Why are the sounds 'ch' and 'j' transcribed as [tsh] and [dzh] in phonetic
alphabets, when neither of these sounds are heard in pronunciation?

2. The same 'ch' and 'j' sound is transcribed as [c] and [inverted f] in Hindi,
even though I find the English and Hindi 'ch' and 'j' sounds undistinguishable.

3. Why does the IPA use the same letters for dental, alveolar and post alveolar
sounds? As a result the English alveolar 't' and 'd' are written [t] [d] in IPA
and the French dentals 't' and 'd' are also written [t] [d] in IPA. Both the
sounds are pretty different.

Can somebody help me by answering these questions?

Nikhil 

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