reporting verbs

hello

I would like to know if these sentences are Ok.

1)'he denied that he had taken sue's calculator'.
2)'she said that he should buy some milk'
3)'he apologised for not coming on saturday'
4)'I suggested him to go back to Singapore'.
5)'I suggested him that he went back to singapore'.
6)'I warned that he didn't take the A 20'.

I would like to know the difference between:
1) 'at which time/point
2) 'in which case'
3) 'by which time'
4) 'in which event'

Thanksss!

[Tdol]

4)'I suggested him to go back to Singapore'. I don't like this one
5)'I suggested that he went back to singapore'.

by which time/at which time- the first is a limit- the thing could have happened earlier- while the second just states the time in the past