Мы оставим антибликовое покрытие, если только рецепт на очки не изменят.

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Breakdown of Мы оставим антибликовое покрытие, если только рецепт на очки не изменят.

• оставить: to keep
• изменить: to change
• если только не: unless

Questions & Answers about Мы оставим антибликовое покрытие, если только рецепт на очки не изменят.

Why is оставим used here, and what tense/aspect is it?

Оставим is the 1st person plural future form of the perfective verb оставить.

• мы оставим = we will leave / we will keep
• Perfective aspect presents the action as a completed decision/result.

Here that fits well, because the speaker is talking about a one-time decision: we’ll keep the anti-glare coating.

If you used an imperfective form, the meaning would change. For example, будем оставлять would sound more like a repeated or ongoing action, which does not fit this context.

Does оставить really mean leave here? Why is it translated more like keep?

Yes. Оставить literally often means to leave, but in many contexts it naturally means to leave something as it is, which English often expresses as keep.

So here:

• Мы оставим антибликовое покрытие
  literally: We will leave the anti-glare coating
• natural English: We’ll keep the anti-glare coating

This is very common in Russian: a verb may be translated differently depending on what sounds natural in English.

What does если только mean here?