суму

In a sentence

• Я відчуваю суму через втрату.

  I feel sadness due to the loss.

  Here, 'суму' is the accusative form of 'сум', meaning 'sadness', used as the direct object of the verb 'відчуваю'.

• Він ще не визначився із сумою покупки.

  He has not yet decided on the purchase amount.

  Here, 'сумою' is the instrumental form of 'сума', meaning 'amount' or 'sum', and specifies the context of the decision.

• Для цієї задачі потрібна точна сума.

  An exact sum is required for this task.

  Here, 'сума' is the nominative form, referring directly to a specific sum or total in a mathematical or financial context.