Unless

The word unless can also create an if-then relationship. But it can be the most confusing and counterintuitive “logic” word. Consider this example:
Your CR score will not be high unless you study formal logic.

There are two ways to translate this statement into if-then clauses:
1) If your CR score is high, then you have studied formal logic.
2) If you did not study formal logic, then your CR score will not be high.

The second method is the foolproof way, which replaces unless with if not. The preferred way is the first method, which negate the clause before unless and cap it with if, and replace unless with then. Basically the clause after unless is the necessary condition which must happen for the negated form of the other clause. Back to the example we have here, “study formal logic” is a necessary step for one to get high score in CR. Without “studying formal logic”, one would not be able to score a high mark in CR. But “studying formal logic” alone might not be sufficient to help you score high in CR.

When the unless-clause comes at the beginning of the sentence, everything between the word unless and the comma is the unless-clause.

Either

The word either can also create an if-then relationship. Consider this example:
Either Peking University or Tshinghua University is on the list of my dream schools.

Given this rule, if PKU is not on my list, then THU is on my list because one of them must be. Further, the rule does not exclude the possibility that both schools are on my list. Therefore, the correct way to say the same thing using if-then clauses is:
If PKU is NOT on my list, then THU is.
Notice the word NOT is added to the if-clause, not the then-clause. Otherwise, we would make the mistaken assumption that both schools cannot be on the list together, which is not necessarily true—at least on the GMAT or LSAT.

Hidden if-then statements

Many if-then statements on the test are hidden because they do not use if or then. Instead, they use words like all, any, when, must and so on. Consider this example:
All Chinese students are diligent.
Translation: If you are a Chinese student, then you are diligent. (I truly hope so!!)

The trick here is that all means if.

There are also words that mean then. Here is another example:
Reading SDCAR’s posts on CR requires good understanding of English.
Translation: If you can read SDCAR’s posts on CR, then you have good understanding of English. (Pat yourself on the back, please!!)

Here are more words you can use to find hidden if-then statements:

If: All, always, any, each, every, in order to, invariably, no, none, things that, those who, to, when.

Then: Depends on, essential, must, necessary, needs, only, only if, only when, prerequisite, requires.

Unless (if not): Except, until, without.

No
When you see no at the beginning of a sentence, change no to if and negate the other clause, which is your then clause. Example:
No one who has a cold should go outside. (No X is Y.)
Translation: If you have a cold, then you should NOT go outside. (If X, then NOT Y.)

Most, some, and not all
Most means more than half. Most could be all.
Some means at least one. Some = Many. Some could be most, could be all.
Not all means some did not. Not all could be none.