盾

1. Origins, structure, and basic meaning

• Structure: 盾 (たて) is a noun for defensive equipment, extended figuratively to a pretext used for avoidance.
• Basic usage: 盾で矢を防ぐ uses a physical shield as an instrument against an arrow, while 規則を盾にする figuratively turns rules into cover for refusing an explanation.

2. Common pitfalls and similar expressions

• Instrument で and figurative result に
  • 盾で marks a real defensive instrument, whereas 盾にする constructs another thing as figurative cover. No physical shield appears in the second example.