Learn French: Il achète de quoi remplir notre grand garde-manger. - He is buying enough to fill our large pantry.

The phrase de quoi followed by an infinitive like remplir is an idiom meaning 'the means to,' 'the things needed to,' or 'enough to.' It implies having the necessary resources (in this case, groceries) to carry out the action.

The word garde-manger looks like a mix of verbs. How is it constructed?

It is a compound noun made from the verb garder (to keep or protect) and the infinitive manger (to eat). Literally meaning 'keep-eat,' it is the standard French term for a pantry or larder.

If garde-manger is a compound noun, how do I make it plural?

In compound nouns made of a verb and an infinitive, both parts are invariable (they do not change). So if you had multiple pantries, it would simply be des garde-manger without adding an -s.

Just as a quick refresher, why does grand go before the noun here?

Most French adjectives go after the noun, but grand is one of the common exceptions that go before it. These exceptions generally describe Beauty, Age, Number, Goodness, or Size (often remembered by the acronym BANGS). Since grand describes size, it precedes garde-manger.