と

1. Origins, structure, and basic meaning

• Structure: Add と after the person or animal accompanying an action.
• Basic usage: It expresses doing something with someone.
  • In 友達と公園へ行きます。, the friend is the companion and the park is the destination.

2. Common pitfalls and similar expressions

• Companion and object
  • In 家族と夕食を食べました。, 家族と identifies the companions and 夕食を identifies what was eaten. The particles have different roles.
• Adding 一緒に
  • In 友達と一緒に行く, adding 一緒に makes togetherness explicit. 友達と行く already conveys going with the friend.
• と in noun lists
  • In 父と母, と joins two nouns. In 父と行く, と marks the companion in an action, so the same character has a different relation to the rest of the sentence.