Learn Spanish (Latin America): Nuestras mochilas son grandes.

The possessive adjective nuestro (our) must match the noun it belongs to in both gender and number. Because mochilas (backpacks) is feminine and plural, nuestro changes to its feminine plural form, nuestras. Notice that while mi and tu only change for number (singular or plural), nuestro changes for gender too!

Why is the adjective grandes not feminine (grandas) to match mochilas?

Adjectives that end in -e in their singular form, like grande (big), do not change for gender. They use the exact same form for both masculine and feminine nouns. To make them plural to match mochilas, you simply add an -s to get grandes.

Why do we use son (are) here instead of están?

We use son, which comes from the verb ser, to talk about permanent traits and physical characteristics. Since being big is a basic, built-in quality of these backpacks, ser is the correct verb to use.

Do possessive words like nuestras always go before the noun?