Thank you Ann,
Would it be totally wrong to say 'a high blood pressure' and 'the screen'?
"A high blood pressure" would not make any grammatical sense in this sentence.
The blood of every living person exerts pressure against the walls of the veins and arteries. This is called the "blood pressure." In some people, this blood pressure is high, so they have "high blood pressure." In other cases, the blood pressure is normal, so they have "normal blood pressure."
"High blood pressure" is a condition, not an object or event. It is like "poor eyesight" or "shortness of breath."
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You would use "the" for screen if it has already been established what screen we are talking about. But if this is the first time anyone has heard anything about screens, then use "a."
"The new library desks offer a new feature --
a noise-abatement screen! The top lifts up to form
the screen, so students are not distracted by ambient noise."
but
"It's so noisy in here. I wish I had a desk whose top lifts up to form
a screen of some kind."