Learn Hardcore Russian: Я закрыл пакет скотчем, чтобы сохранить продукты. - I sealed the bag with Scotch tape to keep the groceries fresh.

Because пакет is the direct object of the transitive verb закрыл. In Russian, direct objects take the accusative case. Since пакет is an inanimate masculine noun, its accusative form is identical to its nominative: пакет.

Why is скотчем in the instrumental case?

The instrumental case expresses the tool or means by which an action is performed (“with,” “by means of”). Here you sealed the bag with tape, so скотчем is the instrumental singular of скотч.

Why is продукты in the plural accusative?

Продукты (“food items, groceries”) is the direct object of сохранить (“to preserve”), so it takes the accusative. In Russian, “food” in the sense of groceries is normally used in the plural, and the nominative and accusative plural forms coincide as продукты.

What is the role of чтобы in this sentence?

Чтобы introduces a purpose clause, equivalent to English “in order to.” It links the main action (закрыл пакет скотчем) with its intended result (сохранить продукты).

Why is сохранить in the infinitive after чтобы, rather than a conjugated verb?

In Russian purpose clauses introduced by чтобы, you use the infinitive to denote the intended action. There’s no need for a subjunctive or additional particles; the infinitive сохранить (“to preserve”) directly expresses the goal.

What is the difference between закрыл and заклеил in this context?

Закрыл пакет скотчем focuses on closing or sealing the bag with tape.
Заклеил пакет скотчем emphasizes the act of sticking/gluing the tape onto the bag.
Both are correct; заклеить highlights the adhesive action, while закрыть highlights the final sealed state.

What is скотч in Russian? Is it a loanword?

Скотч is a loanword from the English “Scotch tape.” In Russian it’s used generically for any adhesive tape, not just the Scotch brand.

How do you pronounce чтобы?

Phonetically it’s [ʂˈtobɨ], roughly “sh-TOH-bi.”

ш = “sh” as in “ship.”
Stress on the second syllable (то).
ы is a close central vowel (no exact English equivalent; imagine “i” in “bit,” but pronounced further back).