One of the most common problems is that students read too passively and superficially. They understand the topic of the passage, but not what or not everything the passage says about that topic. They fill in the missing information with their own opinions and ideas about that topic.
To improve on this,
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another useful exercise is writing paraphrases. Take one of the short (4-5 sentence) passages, and write a paraphrase stating the meaning of each sentence using only your own words, not the words of the original.
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When you have finished the paraphrase, write one sentence stating the main point of the paragraph.
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Make a note of any tone-words and rhetorical devices.
This is what is supposed to be happening in your mind as you read: you explain to yourself the meaning of each sentence in words you can easily understand, then you summarize the main point of the entire paragraph or passage, also in words you can easily understand. Finally, note the tone (angry, cheerful, objective, skeptical, etc) or any special rhetorical devices used (questions, first person, repetition, metaphor, etc.)
If there are words you don’t know, try to guess their meaning from the context. Afterwards, look them up in the dictionary.
Practice with the short passages. When you can handle them well, move on to the long passages. For the long passages, you can complete the paraphrase/main point/tone-rhetorical device analysis out loud. Practice with long passages until you can complete them rapidly. Then proceed silently.
Many or even most of the reading comprehension questions can be answered easily if you are able to paraphrase the text in your own words, determine the main point of paragraphs and passages, and identify the tone and rhetorical devices. There are also some questions that involve determining point of view. Usually, if you have understood the meaning and tone well, the point of view is clear.
Having said all this, you should note that if you are not already a strong reader, it takes some time to get to the point that you can do all the required mental operations in the time available during the SAT. It is normal to get stuck at a score level for several months (or even longer). Speed comes with practice. You want to improve your natural speed, not to rush. Rushing leads to falling into traps. Reading high complexity books at least an hour per day helps to develop a faster natural reading speed, so try to add as much high complexity reading as you can to your daily routine. However, even if you just practice methodically with passages, you will definitely improve eventually, and if you persist a lot, you will improve a lot.