The simple explanation is that an item is shown as critical if changing the duration would affect the completion date.
If you had a programme with 2 tasks, a and b, that occur concurrently. where a took 5 days and b took 3 days, a would show as critical, as any change to the duration would affect the completion date. If task b took 1 day or 4 days, it wouldnt matter to completion.

The critical path is not something you make, it comes out of the programme. Trying to manipulate a programme to manufacture a CP is never, ever a good idea- it means that tasks will have incorrect duration, incorrect links, or worse.

Think of it as Project telling you what items have to be done on time to avoid failing. as you change the programme the CP will change to reflect these changes.