Learn French: Ce chien est très gros.

You would change the masculine chien to the feminine chienne. Notice how it doubles the n and adds an e, which perfectly matches the rule you are learning! You also have to change the rest of the sentence to match: Cette chienne est très grosse.

Why did gros change to grosse for a female dog?

Gros (big) is an irregular adjective. Just like the rule you are learning for words ending in -en or -on, you double the final consonant to make it feminine. The s doubles and adds an e, becoming grosse.

Why do we use ce here instead of cet?

Both mean 'this' or 'that' for masculine words, but you only use cet if the next word starts with a vowel or a silent h (like cet animal). Since chien starts with a consonant, we just use ce.

What is the difference between gros and grand?

While both can translate to 'big', grand usually describes being tall or long in size. Gros describes volume, thickness, or being overweight. So this dog is bulky or fat, not just tall!