要る

1. Origins, structure, and basic meaning

• Structure: 要る is read as いる and is a verb meaning that an item or condition is needed or required.
• Basic usage: Mark the needed item with が in 細い筆が要る, and use the negative 傘は要らない to contrast an unnecessary item.

2. Common pitfalls and similar expressions

• が and は
  • が introduces the brush as what is needed, while は makes the umbrella the topic and contrasts it as unnecessary.
• Spelling and negative form
  • The need verb 要る differs in spelling from 居る, which expresses presence, and its negative form is 要らない.