Transitive and Intransitive Verbs:
Transitive and intransitive verbs are two types of verbs that are used to describe the action or state of being in a sentence. The main difference between these two types of verbs lies in their ability to take a direct object.

Transitive Verbs:
A transitive verb is a verb that requires a direct object to complete its meaning. In other words, it is an action verb that transfers the action onto the recipient or the object of the verb. The direct object receives the action of the verb and answers the question "what" or "whom" after the verb.

Intransitive Verbs:
An intransitive verb is a verb that does not require a direct object to complete its meaning. It is an action verb that does not transfer the action to anyone or anything. Intransitive verbs typically express a complete thought without the need for a direct object.

Explanation:
In the sentence "You must speak the truth," the verb "speak" is used transitively.

Transitive Verb:
The verb "speak" requires a direct object to complete its meaning. In this sentence, the direct object is "the truth." Without the direct object, the sentence would be incomplete and the meaning would be unclear.

Example:
- You must speak (incomplete)
- You must speak the truth (complete)

Intransitive Verb:
If the sentence were written as "You must speak," without the direct object "the truth," the verb "speak" would be used intransitively. However, in the given sentence, "the truth" is necessary to complete the meaning of the verb "speak."

Conclusion:
Therefore, in the sentence "You must speak the truth," the verb "speak" is used transitively because it requires a direct object ("the truth") to complete its meaning.