Hi mes08!
Prephrasing can be tricky at first! I'll start by linking you to some great posts on prephrasing that Jon wrote to help you practice it:
Part I is here.
Part II is here.
I'll also give you some tips about prephrasing specifically for Must Be True, Strengthen, Weaken, and Justify:
Must Be True
For Must Be True questions, sometimes you'll be able to prephrase specifically and sometimes you won't. When you have conditional reasoning, you can often prephrase specifically by taking the contrapositive of the conditional statement or by linking multiple conditional statements together. When you don't have conditional reasoning, you may not be able to prephrase as specifically. Instead, make sure you understand each of the facts presented. Is there a pattern being presented over several facts? Is there a statement that is strangely worded or particularly tricky?
You won't always be able to come up with a specific prephrase because there are usually multiple things that Must Be True and you can't always anticipate exactly what they're going to give you in the answer choice. But if you know the facts well and have looked for any connections you can make between them or tricky points to focus on, you'll be in good shape going into the answer choices.
Strengthen/Weaken
For Strengthen and Weaken questions, look for gaps or flaws in the argument. If you can find a gap/flaw and it's a Strengthen question, then your prephrase is that you're looking for something to fix that gap/flaw. If it's a Weaken question, your prephrase is to aim your attack at the gap or flaw. If there's a clear missing link between information in the conclusion and information in the premises or a clear flaw in the author's reasoning, then you should be able to come up with a pretty specific prephrase.
If you can't find a gap or flaw, remember to always focus on the conclusion. Sometimes it helps to just identify the conclusion and then think backwards to the premises the author used to reach that conclusion. Do the premises really add up to that conclusion?
When it seems like the premises do add up to that conclusion, focus on the conclusion. Let's say my conclusion is: "The tax increase is harmful." My prephrase for something that would Weaken that conclusion is simply to find an answer choice that indicates a way in which the tax increase might actually be beneficial. If I want to Strengthen that argument, I would look for an answer choice that would show a way in which the tax increase is harmful. There are probably multiple ways in which the tax increase could be harmful/beneficial so I wouldn't prephrase more specifically than that.
Finally, remember that if you have causal reasoning, your prephrase is that you're looking for one of the 5 ways to Strengthen (eliminate an alternate cause, cause with effect, no cause no effect, eliminate reverse cause and effect, support the data) or Weaken (alternate cause, cause without the effect, effect without the cause, reverse cause and effect, problem with the data) a causal argument.
Justify
For Justify questions, look for the missing link in the argument. Most of the time, you'll be linking new information in the conclusion back up to the information in the premises so you can often prephrase specifically. Again, sometimes it helps to identify the conclusion and then think backwards to how the author got there. What else do you need to add to the premises to prove that conclusion 100%?
Hope this helps!
Best,
Kelsey