'Have had' should be used when talking about yourself. 'I have had that.'
It can also be used when talking about a group. 'They have had that.'
'Has had' is when talking about another person. 'She has had that.'
(Sorry about to horrible examples, I'm too tired to sit and think of good ones)
Yeah, that is fine.
You should use "are".
You should use did.
you should use meters
You should use your treadmill on a regular basis. You should use it every day if possible. You should use the treadmill for twenty to thirty minutes.
you should use you all
There are number of different kinds of gas you should use. The gas you should use depends on the situation.
Why should you improve your ability to use the library?
You should not. You should use centimetres.
we should use had if the sentence is in past tence
if we made mistake
you should use spagetti and sauce