Use borrow when you want to express the act of receiving something from someone else with the promise to return it later. It highlights the temporary nature of the transaction and the responsibility of the borrower to give back what was taken.

The term lend is best used when you are offering something of yours for someone else's temporary use. It implies a generosity or helpfulness on part of the lender, as well as an expectation to recover the lent item.

In brief, borrow and lend are two sides of the same coin, focused on the temporary transfer of something. Borrow centers on the recipient's perspective and emphasizes the act of taking something temporarily, while lend regards the act from the giver's perspective, emphasizing the provision of something for temporary use. Both terms imply the eventual return of the borrowed item to its original owner.