Can you explain how HN39 'knew' your pilot training history since you appear to have no posts before this thread.?
Now then:
TAS will increase with altitude and are the speed witch the airplane moves through the air.
Second part correct. Let's keep this really simple. Say you are climbing at Mach1 all the way (NB that is a 'CONSTANT MACH NUMBER' - yes?). At sea level (and any level) TAS = speed of sound. Agreed? You say (correctly) that the speed of sound DECREASES with altitude. So, is your TAS at Mach1 higher or lower at 30,000ft than at sea level?
My understanding of IAS is that it does not change with altitude since it measures the difference between static pressure and dynamic pressure. I think we will ignore that as it complicates things. What you should understand as an instructor is the way IAS changes as you climb at a constant TAS. Google it if you need to. It applies to Cessna 152s as well as jets!
Let's sort TAS/IAS first then look at L/D.
PS Amongst your 'library', do NOT put too much store by 'Ace the technical Interview' - it has had very bad reviews on PPRune.
Both Mach and TAS increase with altitude,
- this is mind-bogglingly confused! How can Mach 'increase' if you are climbing at a constant Mach number?