Formal or informalBuy or purchase
Everyday: speech, shopping, chats.
Let's buy samosas after school.
Formal: receipts, notices, business letters.
Tickets may be purchased at the counter.
Remember: Say buy to a friend. Notices and invoices say purchase. Saying "purchase" at a chai stall sounds stiff.
Short lessonFormal or informalWe speak differently to friends, teachers and officials. This change in style is called register.