Quando a prateleira estiver cheia, compraremos outra.

When the shelf is full, we will buy another one.

Breakdown of Quando a prateleira estiver cheia, compraremos outra.

• estar: to be
• comprar: to buy
• quando: when
• cheio: full
• a prateleira: the shelf
• outro: another one

Questions & Answers about Quando a prateleira estiver cheia, compraremos outra.

What tense and mood is estiver? Why is it used after quando?

estiver is the third-person singular future subjunctive of estar. Portuguese uses the future subjunctive in subordinate clauses introduced by conjunctions like quando when referring to actions or states that will occur in the future but are not yet realized.

Why can’t we use está (present indicative) or estará (future indicative) instead of estiver?

Using está would imply a general or habitual fact rather than a future condition. estará isn’t allowed after quando; Portuguese grammar mandates the future subjunctive in such temporal clauses to express an uncertain future event.

Why is compraremos used here? Could we say vamos comprar instead?

compraremos is the simple future indicative (“we will buy”). You can use vamos comprar (“we are going to buy”) in informal speech:

• “When the shelf is full, we’re going to buy another one” → Quando a prateleira estiver cheia, vamos comprar outra.
  Both convey the same idea, but compraremos is more formal and common in writing.

What does outra refer to, and why is it feminine?

outra is a feminine singular pronoun meaning “another one.” It agrees in gender and number with prateleira, so outra stands for outra prateleira.

Why is there a comma after cheia? Is it always required?

When a subordinate clause introduced by quando comes before the main clause, standard punctuation places a comma between them. If you invert the sentence—Compraremos outra quando a prateleira estiver cheia—the comma is optional and often omitted.

Why is there a definite article a before prateleira? Can it be dropped?

Portuguese generally requires the definite article before singular countable nouns when referring to a specific item. A prateleira means “the shelf.” You can’t drop it (Quando prateleira estiver cheia is ungrammatical). To speak generically, you’d use the indefinite article: quando uma prateleira.

Where is the stress in prateleira, and why isn’t there an accent mark on ei?

The stress falls on the penultimate syllable, -lei-: [pɾɐ.tɐˈlej.ɾɐ]. Words ending in a vowel normally have stress on the next-to-last syllable, so no accent mark is needed. The ei is a diphthong following the default stress rule.