The DET shows you a short passage in which many words are missing their second half, and you type the missing letters. It appears in no other English exam, so students meet it for the first time in the test unless they have practised it deliberately.
It is a grammar and collocation test wearing a spelling costume. The letters you are given narrow the possibilities; the sentence decides which one it is. "The stu____ were asked to sub___ their ass_______ by Friday" has exactly one sensible reading, and none of it depends on vocabulary you do not already have.
Work in this order. Read the whole sentence first, ignoring the gaps. Then fill the ones you are sure of — usually function words (the, that, with, their) and word endings (-ing, -tion, -ment, -ness). Those anchors make the harder gaps readable.
Mind the grammar of the ending. A gap after "they" needs a plural verb; a gap after "has" needs a participle. The letters shown will fit several words, and the grammar picks between them.
Type carefully. A correct word with a typo scores as wrong, and this is a timed item where the instinct is to rush. Slow hands, fast eyes.
Where the practice comes from. Take any paragraph and delete the second half of every third word. Restore it a day later, so you are reading rather than remembering. Fifteen minutes of that a day for a week visibly changes your score on this item.