Hi,

1. When you use the word 'when', do you have to use a past perfect in the subordinate clause to denote the past activity or can we use a past there like the main clause to make the context apparent?

When he had eaten his lunch, he rose up got up and went home.

When he finished (or 'ate')  his lunch, he rose up got up and went home.

The above is OK with Simple Past, because the sequence of events is clear.

An example of a case where the Simple Past does not make the sequence clear is 'When he ate his lunch, he read a book'. If you want to show that the eating was first, you can say 'When he had eaten his lunch, he read a book'.

2. Why is these tenses are not correctly used?

Workers are installing extra loudspeakers because the music in tonight's concert will need amplication. This is OK.

Can I make it like this?

Workers are installing extra loudspeakers because the music in tonight's concert will be???needing amplication. This is OK, if you feel that you want to stress the duration of the 'needing'.

Best wishes, Clive