Dogs talk all day, just not in words. The trouble is that people tend to read one signal, usually the tail, and miss the rest. A tail can wag when a dog is thrilled, but also when it is unsure or on edge. The fix is to read the whole dog at once: ears, tail, eyes, mouth and body, together, in the situation it is in.
Below is a small reader. Pick what you can see on a dog, and it tells you what that combination usually means and what to do next. It is a guide to the common signals, not a diagnosis of any one dog.
Relaxed ears sit in their natural spot for the breed. Ears pricked forward mean the dog is focused on something. Ears pinned flat back are a common sign of fear or stress. Some breeds have ears that are naturally floppy or folded, so learn what "normal" looks like for your dog.
Speed tells you how aroused a dog is, not whether it is happy; fast wags can show excitement, but they can also show tension. Height adds to the story: a neutral tail is usually friendly, a stiff high tail is alert, and a tucked tail points to fear. Some dogs, such as greyhounds, naturally carry the tail low, and curled or docked tails show less, so don't lean on the tail alone.
Soft eyes and a loose mouth are relaxed. Lip-licking, yawning when the dog is not sleepy, and a panting mouth with a curled tongue tip can be signs of stress. "Whale eye" is when you can see the whites of the eyes as the dog looks sideways without turning its head, and it is another stress signal.
A loose, wiggly body is a comfortable dog. A frozen body, weight leaning forward, means the dog is concentrating hard on something and may be unsure. Crouching, trembling, or backing away says "I want out".
Ask what just happened. A dog that crouches with its bottom in the air and front legs down is probably doing a "play bow", an invitation to play, and it is normally paired with a loose, bouncy body. The same dog freezing next to a stranger is saying something very different.
Read the whole dog, not one body part. When in doubt, give space and let the dog decide.
Stay calm, and don't raise your voice. Move the dog (or the trigger) so there is more distance, and give a quiet place to retreat to, such as a crate or bed in a calm room. Reward relaxed behaviour with treats and praise, which builds a good feeling about whatever worried the dog. Don't force a dog to "face it"; going slowly is faster in the long run.
Don't punish growling. A growl is a useful warning, and a dog that has learned not to growl may skip straight to a bite. Sudden changes in behaviour, a dog that now snaps when touched, or loss of appetite, trembling or toilet changes can be a sign of pain or illness. That is a job for your vet, not a guess from a guide.
Sources: Blue Cross, signs of stress in dogs and VCA Animal Hospitals, interpreting tail wags in dogs. If your dog's behaviour worries you, ask your vet.