Must also be true prompts you to extrapolate a new statement using formal logic based on information presented in a stimulus.
This task is prompted by a fairly pedestrian question stem, as exemplified by the explanations for the October 1996 Sample PrepTest below. On modern LSATs, the most common version of this question stems is:
If the [stimulus] above is true, which one of the following must also be true?
Two general structures that Zen of 180 students use to visually represent the must also be true task
In the example on the left, the credited answer (C), is a necessary condition interpolated between two or more pieces of evidence presented in the stimulus (A and B), i.e. in order for B to be true in the context of A, C must also be true. In the diagram on the right, the credited response is a combination or two or more pieces of evidence from the stimulus, that when combined lead to a new conclusion.
The key to correctly answering a must also be true question is to clearly highlight the main pieces of evidence--the actors and their definitions--and how they can be combined in terms of topic, degree, certainty, and opinion. As with depends upon assuming, the goal is not to describe the specific evidence--as the LSAT answers will invariably mention the "correct" parts--but rather the links in relationship and the degree of certainty between the pieces of the stimulus.