I was watching this video which says (at 0:55) we cannot use "will" to describe future plans because plans are not certain and we only should use "will" when we are certain about the happenings that take place in the future to some extent.
It contradicts my previous knowledge. Is this claim correct?
This video is a little bit right and a lot wrong.
It's a little right in that we don't use "will" to indicate that we're talking about firm plans. "Will" is for believed or predicted future events.
It's dead wrong to say we only use "will" "when we are certain". "Will" is used for uncertain future, especially with hedges like "I think" and with conditional clauses like "if it rains". We use present continuous to refer to firm future plans.
"Will" is also used for predictions of the future in general, with no regard for whether this future is based on someone's plans or not:
The sun will rise at 5:43 am tomorrow.
The laundry will be finished in an hour.
Same goes with:
All the factories in this area will be removed in the redevelopment.
That sentence is 100% correct, along with the "be going to" variant. So that video is dead wrong.
"Will" is generally used for statements of fact or high certainty statements about the future.
The sun [will/should/might] rise at 5:43 am tomorrow.
You use "will" if you're certain, "should" if you're fairly certain and "might" if you're uncertain (in addition to other words you can use).
If it rains, the ground will be wet.
For the above, there is certainty that the ground being wet will be the case if it rains. But you're not expressing whether you're certain it will rain.
I think the sun will rise at 5:43 am.
For the above, you're presenting a statement of fact ("will") and saying you're uncertain about that fact ("I think").
So the example given is perfectly fine:
All the factories in this area will be removed in the redevelopment.
Similar to the "if it rains" example above, this is conditional on the redevelopment taking place ("in the redevelopment"). Even if whether the redevelopment will take place is not certain, you can still use "will" for things that will happen in the redevelopment if you add "in the redevelopment".
I watched the first part of that video. The speaker is simply wrong, in several areas.
First, let's get this out of the way: There are formal and constrained versions of language for writing contracts or specifications. In that case, you would usually define your terms. For example, pulling a NASA example found via Wikipedia:
Use imperatives correctly and be consistent. Remember, shall "prescribes", will "describes", must & must not "constrain", and should "suggests"
In contracts and specs I have personally worked on, I instead usually see that "shall" indicates a requirement and "will indicates simple futurity" (meaning that it's just the future tense).
Regardless, if there is some standard governing your language, you follow that standard. If you are writing language intended to be legally binding, cite a standard or define your terms.
The above probably doesn't apply to you. Here's what does:
When you are writing, use "plan" only when you have some structured course of action in mind. There is a saying that a plan is a "dream with a deadline," meaning that the concrete deadline is what distinguishes a plan from an idle dream. Weaker versions might be "we intend to...", "we are considering...", "we want to...", etc. However, don't assume that other people are speaking precisely. "Plan" is one of those words which can mislead you.
"Will" is highly context dependent. If you are describing a rocket launch sequence, or some other highly choreographed situation, "will" comes with very high certainty. However, it is also used in the same sense when you are describing your vision of future events ("In aa hundred years, everyone will have a flying car.") As such, "will" is perfectly fine as a verb for this use.
However! "Will" by itself is not idiomatic. The common idiomatic version is to contrast the "as-is" (current situation) with the "will-be" (envisioned future situation). If the presentation is intended to have a slightly more directive tone, "will-be" can be replaced with "to-be".
Here is an example of that use, applied to your map. I am not saying this is the best description. I am only demonstrating this use. I'm using complete sentences, but you would more often see terse bullet points used here:
As-is: Norbiton is only suitable for industrial use.
- Most buildings are unsuitable for residences.
- There is limited space to expand the development.
- There are no local amenities.
Will-be: Norbiton is a residential community.
- Industrial buildings are converted to residential or replaced.
- A new bridge allows expansion across the river.
- A new school and playground have been built, and space has been allocated for a medical building and light commercial area.
You might see this use as part of a presentation (like next to the picture from the exercise!), but you generally would not see it in the middle of a paragraph.
People may be interpreting the video in a way that wasn't intended. There's another way to look at it and the document it's based on. The intended focus may not have been on whether this use of "will" is grammatically incorrect or violates its meaning.
You can't ignore the fact that nothing in the future is certain. The degree of certainty is not about the absoluteness that "X" will happen, but a characterization of your belief that it will happen. That assessment of certainty must be based on conditions and assumptions (what happened in the past, what plans are in place, what forces might be aligned to help or hinder, etc.).
Any rule where the use of "will" is based on the degree of certainty is arbitrary. There is no real definable point of certainty, and the meaning will be different for each person. I think a different point is being made.
The image in the question shows a comparison of different ways to express the idea. "Will" (or "be going to") just states your firm belief with no context. The checkmarked options frame it as plans, which is more nuanced, precise, and informative.
The takeaway is that those options are a better practice if you need to pick words to describe planned actions. That doesn't mean that the use of "will" in that situation is incorrect.
