借りる and 貸す: borrower's and lender's viewpoints
Describe one transfer from either the borrower’s or the lender’s viewpoint. Check how the role of a person marked by に changes with the verb.
Describe the same event from two viewpoints
私は佐藤さんに辞書を借りました。 says that I borrowed a dictionary from Sato. The borrowed item takes を, while 佐藤さんに identifies the lender. から can be used instead of に to emphasize the source.
佐藤さんは私に辞書を貸しました。 describes the same event from the lender’s side. Sato lends the dictionary and 私 receives it. Choose 借りる or 貸す according to whether the subject is the borrower or the lender.
Read に together with its verb
With 借りる, person plus に marks the source of the item. With 貸す, person plus に marks the recipient. Compare 私は兄に本を借りた。 with 私は弟に本を貸した。 Although both sentences contain に, the direction is opposite.
図書館で本を借りました。 uses で for the place where the borrowing took place. 友だちから本を借りました。 uses から to state the source. Keep the location marked by で separate from a source marked by に or から.
A current loan and the act of returning
私はこの本を友だちから借りています。 can mean that I currently have this book on loan from a friend. この本は友だちに貸しています。 means that the book is currently lent to the friend, so the viewpoint is reversed.
Use 返す for returning a borrowed item. 昨日借りた本を今日返します。 states the borrowing and the return separately. 貸す does not mean giving an item back to its owner.
Quick check: In 田中さんは私に傘を貸してくれました, who lent the umbrella?
Show the answer and why
Tanaka did. 田中さん performs 貸す, while 私 receives the umbrella. From my viewpoint, the same event can be expressed as 私は田中さんに傘を借りました。