What is the infinitive form of the verb used in the sentence "it is restful _____ down on a bed"?

The infinitive form of the verb used in the sentence "it is restful _____ down on a bed" is "to lie." This is because "to lie" refers to reclining in a horizontal position, which fits the context of the sentence that implies resting on a bed. The usage focuses on the action of assuming a position for relaxation or sleep.

In contrast, "to lay" means to place something down and requires a direct object, which doesn't suit the context of the sentence where the action is not about placing but rather about the act of resting oneself. "To rest" is also not applicable here, as it denotes a state rather than the action of reclining. "To sleep" does imply a related action but does not fit grammatically into the specific sentence structure provided. Thus, "to lie" is the appropriate choice for conveying the intended meaning of reclining.