[JonPR]

Originally Posted by ameetnsharma

The IV chord may last a while depending on the piece of music right? maybe the bridge of a song is all on the IV chord.

True. It's not common, but it certainly possible.
What is also possible (and maybe a little more common) is a modulation to IV in the bridge: meaning the scale becomes F major for that section.
But it's true that if the key centre feels (sounds) like F, and you use the C major scale, then you have (in the modern sense) F lydian mode. It's not out of the question for that to happen in the middle of a C major composition, but is still uncommon (in my experience).
My argument is really on the basis of common practice. It doesn't rule out exceptions. It's important to understand how music works most of the time, and then to use the most appropriate terminology for those common practices. That helps us explain the exceptions, using other terms.

My other angle on this is that, before the 1960s, no jazz musician had ever heard of modes. They improvised perfectly well on key-based music, with no inhibition, and had all the terminology they needed. Thinking in modes would not have helped them, and they would have regarded it as irrelevant terminology. It would have added nothing to what they knew, or to how they played. They already used all 12 notes, with a proper understanding of tonal hierarchy from moment to moment.
When the "modal jazz" revolution came along (beginning with Milestones and Kind of Blue), then things changed. That's when this notion of "chord-scale theory" began to be developed, to deal with the kind of sequences where every chord might imply a different key, and where there often was - in a real sense - no key at all.
No problem there. New terms are needed to describe new forms of music. The problem comes when those new terms - that new perspective - is applied to older forms of music; music based on major or minor keys - which still exists and is still widely popular. Jazz musicians may not have a problem because they understand the distinction; they would be consciously applying a new perspective to music that they know predates that perspective - because a lot of what jazz is about is rethinking the past, updating it, taking it new places.
But it is a problem - because it causes endless confusion - when those learning and using the terms don't understand that there is a difference. Some music is key-based, some music is modal. Chord-scale theory applies properly to the latter, not to the former.
You can apply it to the former, but (a) it makes improvisation a lot more complicated, and (b) (unless you know what you're doing) it risks destroying its coherence.

However, the situation IS inevitably complicated in practice, because most contemporary music - in rock as well as in jazz - is frequently a mix of both concepts: keys and modes in the same tune. Or rather, a key with a lot of modal influence; or a mode treated as if it's a key.
Even so, it's still good to be aware of the distinction - and it's also true that traditional concepts of improvisation (dating back maybe 100 years or more) will still work in any kind of modern music. We don't need chord-scale theory, even for modal music. (It can help, but is not essential.)

Of course, at the same time, we can't expect old-fashioned key-based (diatonic) theory to apply in modern music (esp rock). Eg, it's silly (or maybe just naïve) to look at a rock song and point out chords that "break the rules" (such as a bVII in a major key). That's just misreading the rules, or not understanding the flexibility of the rules. No good music ever breaks any rules; it just might be following rules you don't yet know about.

In short, my whole purpose here is to call stuff what it is. It's not about saying you "can't" do this, or you "must" do that. We all do what we think sounds good: that's the only rule. This kind of debate is about suggesting what we ought to call those sounds - and ought not to call them - in order to clarify best what's going on.

Originally Posted by ameetnsharma

Relating to condition 1 above... over the F(IV) chord... to emphasize F tonality, don't you think starting and ending phrasing on F makes any difference? As compared to say starting and ending phrases on C?

Of course, but it isn't a modal difference. The mode is the same regardless of what note you start on, what note you emphasise, or what scale pattern you use.

If you have an F tonic/keynote/tonal centre (not just an F root in a C major progression), and you use the C major scale, "F Lydian" is what results, inevitably. Different emphases, phrasing, target notes, etc, all have different effects, but they're not modal ones.
If you choose to avoid the B entirely, then the mode will simply be ambiguous - could be Ionian or Lydian; in fact we'd probably assume Ionian because that's the more common sound of the two; playing a Bb will confirm that expectation. That's why hitting the B has the noticeable "Lydian" effect.
...

EDIT: btw, check out two of my earlier posts in this thread, #20 and #30 (two years ago!) for some examples of music with clear modal sounds. Mostly the musicians concerned didn't know they were using modes, but we can still use those terms to describe what they did, to help differentiate the sounds from traditional key-based ones.