Questions & Answers about У нас было пять коробок и шесть банок.
Why are the words for "boxes" and "jars" spelled коробок and банок? Where did the extra о come from?
The dictionary forms are коробка (box) and банка (jar). Since both are feminine nouns ending in -а, their Genitive plural form drops the -а (giving them a "zero ending"). However, this leaves two consonants at the end (-бк and -нк), which is hard to say. To make it easier to pronounce, Russian inserts a "fill vowel" (usually о or е) between the consonants.
Why do we use the Genitive plural here instead of the regular plural?
In Russian, numbers 5 and above (like пять and шесть) require the noun that follows them to be in the Genitive plural case. If it were 2, 3, or 4 boxes, we would use the Genitive singular instead (две коробки).
We had multiple boxes and jars, so why is the verb было (neuter singular) instead of были (plural)?
Just as a refresher, why doesn't the sentence use мы for "we"?