Let's look at your sentence: When a goose gets grumpy (and they often do), it hiss to tell everybody it isn't happy.
There are a couple of small grammar mistakes here:
- "it hiss" should be "it hisses" because when we talk about what one goose does, we add an "-es" to the verb "hiss".
- The sentence mentions "a goose" which is one, so we use "it" instead of "they". So, it's better to say "(and it often does)".
Here's the corrected sentence:
When a goose gets grumpy (and it often does), it hisses to tell everybody it isn't happy.