Eight words you already know, like decide, believe and remember, and the three-second check that tells you which form a sentence needs.
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No spelling mistakes. No grammar mistakes. Every sentence is still wrong.
Read the gap in the sentence before you choose the word. Three seconds, three questions.
You know decide. You know understand. You know believe.
So why does the examiner mark them wrong?
Because a word is not one word. It is a family, and each member of the family fits a different place in a sentence.
decide decision decisive
(the action) (the thing) (the description)
Write the government must make a decide and you have used the action word in a place that needs a thing word. The reader understands you. The examiner marks a word-formation error.
A sentence is a row of holes. Only one shape fits each one.

The slot chooses the form. You only have to read the slot.
Eight gaps. Say what kind of word each one needs — no word choice yet.
Terms
Definitions
Three decisions, made in the right order: name the slot, then pick the word.
Chain 1
Sentence: Her ______ was correct.
Candidates: decide / decision
The slot needs:
So the answer is:
Chain 2
Sentence: His anger was completely ______ after what happened.
Candidates: understanding / understandable
The slot needs:
So the answer is:
Chain 3
Sentence: Fleming's ______ of penicillin changed medicine.
Candidates: discover / discovery
The slot needs:
So the answer is:
Eight sentences, one family each. The family is given — the form is yours.
My ______ of the question was completely wrong. (understand)
It was the most ______ evening of the whole trip. (remember)
The committee must reach a ______ before Friday. (decide)
His story was simply not ______. (believe)
The ______ that she had misread the timetable came too late. (realise)
She is an unusually ______ designer. (imagine)
______, most students said they wanted more feedback. (understand)
The government acted ______ once the figures were published. (decide)