To throw the spotlight onto one idea, lead with it, then add is / was what + the rest: Innovation is what will decide this. Next to the plain "Innovation will decide this," fronting the idea makes it feel like the one decisive thing — so reach for it in persuasive or argumentative writing and speech when you want a single point to land hardest. Use what here, because it already means "the thing that". That cannot do that job alone — it needs a noun first: Innovation is the factor that will decide this works, Innovation is that will decide this does not. The same split also runs backwards, as What will decide this is innovation, which saves the focus for the end. Same pattern either way: pick the order that puts the weight where you want it.
Put the key idea first: X is what ...
We need more funding most. → Additional funding is what we need most.
Your support made this possible. → Your support is what made this possible.
Innovation will decide our success. → Innovation is what will decide our success.
Not "Additional funding is that we need." — say "Additional funding is what we need.". This pattern uses what (the thing that), not "that".
Try it: I value your honesty most. Lead with Your honesty and spotlight it. (Leave out the word you don't need.) Answer: Your honesty is what I value most. — Leading with Your honesty and spotlighting it gives Your honesty is what I value most. The link word is what (it means the thing that), so "that" is left out. The key idea moves to the front and what carries the rest.
Clear up what's NOT the case: It wasn't X that ...
To knock down a wrong assumption before giving the real one, use It wasn't / isn't + X + that ...: "It wasn't the budget that was the problem, it was the timeline." It is an emphatic corrective — reach for it when someone has fixed on the wrong cause and you want to deny it and replace it in one breath, which is why it is common in debate, explanation and pushback. After the spotlighted thing, use that, not "what".
The timeline was the problem, not the budget. → It wasn't the budget that was the problem, it was the timeline.
The quality concerns me, not the cost. → It isn't the cost that concerns me, it's the quality.
The weather caused the delay, not the staff. → It wasn't the staff that caused the delay.
Not "It wasn't the budget what was the problem." — say "It wasn't the budget that was the problem.". After the spotlighted thing in this pattern, use that, not "what".
Try it: The timing put them off, not the price. Spotlight what was NOT the cause: the price (it was the timing). (Leave out the word you don't need.) Answer: It wasn't the price that put them off. — After the spotlighted thing (the price), this pattern uses that: It wasn't the price that put them off. "What" is the wrong link word here, so it is left out. The negative cleft clears up what was not the cause.
It is for this reason that ... (spotlight a phrase)
To spotlight a longer phrase — a reason, time, or manner — lead with It is / was + the phrase + that, then the rest. This is a formal, weighty way to foreground the phrase: It is for this reason that the policy was changed loads far more stress onto the reason than the plain "The policy was changed for this reason." Reach for it in academic, formal or rhetorical writing when you want the reason, moment or manner to carry the emphasis and to sound deliberate. After the spotlighted phrase the link word is that, not "what".
The policy was changed for this reason. → It is for this reason that the policy was changed.
We understood the risk only later. → It was only later that we understood the risk.
Real leaders emerge in times of crisis. → It is in times of crisis that real leaders emerge.
Not "It is for this reason what we must reconsider." — say "It is for this reason that we must reconsider.". After the spotlighted phrase, use that, not "what".
Try it: We must reconsider, and this is the reason. Build the formal opener: It is for this reason that ... (Leave out the word you don't need.) Answer: It is for this reason that we must reconsider. — The formal opener is It is for this reason that: It is for this reason that we must reconsider. After the spotlighted phrase, the link word is that, so "what" is left out. This pattern lets you point firmly at the reason.
Try it: A colleague is sure the high cost sank the deal — but really it was the slow delivery time, and you want to correct them. Which cleft does that job best? (It wasn't the cost that sank the deal — it was the delivery time. / The delivery time is what sank the deal. / It is for this reason that the deal sank.) Answer: It wasn't the cost that sank the deal — it was the delivery time. — Your job is to deny a wrong cause (the cost) and supply the real one, so the negative cleft It wasn't the cost that ... — it was ... is the tool: it knocks down the mistaken idea and replaces it in one move. The delivery time is what sank the deal is a fine reversed-pseudo cleft, but it only spotlights the true cause and never corrects the wrong one. It is for this reason that the deal sank is a formal opener that points at a reason you haven't even named — weighty, but not a correction.
Try it: You're closing a persuasive talk and want one idea — innovation — to feel like THE deciding factor. Which cleft lands it hardest? (Innovation is what will decide our future. / It wasn't innovation that will decide our future. / It is for this reason that innovation matters.) Answer: Innovation is what will decide our future. — To make one idea feel decisive in persuasive speech, front it with the reversed-pseudo cleft: Innovation is what will decide our future puts innovation first and lets what carry the rest. It wasn't innovation that ... denies innovation — the opposite of what you mean. It is for this reason that innovation matters is a formal opener that leans on a reason you haven't stated, so it doesn't spotlight innovation itself.