You know how, as humans, we automatically pick up on each other's nonverbal cues? We can tell someone is uncomfortable when their arms are crossed or agitated when they tap their foot. Well, dogs give off non-verbals, too! When a dog wags their tail, we all know that means they're excited. But there are a lot of other non-verbal cues dogs give us to better understand them, some you've probably never even thought about!
Tails - As just said, we know a wagging tail means a happy pup. But their tail can tell you other things, too. If the wagging is tight or twitchy, that is a wag out of nervousness. If your dog is frightened, their tail can drop or go right between their legs.
Lips/Mouth - Dogs can't physically smile like we can, but you can tell when they want to! A dog having a parted mouth means they are happy and relaxed. If their jaw is tensed or their mouth tightly closed, they're in their own head, thinking! And when your dog snarls or shows teeth through a lip curl, that means they feel threatened and might act aggressively.
Ears - Ears up, ears down! Watch your dogs ear flick (even floppy ears flick where they connect to your pup's noggin) to see which direction their ears are facing. If they're facing you, you've got your dog's attention! Whatever your dog is concentrating on, their ears will flip towards that. Ears flat back against their head means they feel frightened and are trying to protect themselves.
Posture - Human and dog posture are not all that different. Just like us, a dog will have a taller, straighter stance if they are excited or happy. But if they have sunken shoulders or are hunched over, that means they're feeling overwhelmed.
So remember, humans aren't the only ones to give non-verbal cues! If you're ever unsure of how your dog is feeling about something, just observe their body (specifically their posture, tail, mouth and ears) to give yourself some hints.