When kids learn language, they start by associating sounds with objects or ideas. For instance, if a child hears the word "bottle" every time the child is presented with a bottle, the child will eventually learn to connect the sound of the word with the object. In this way, kids understand words before they learn to express them. One could say the same thing happens with dogs. Dogs just never make the next step to speak with words. However, whether a dog's "understanding" of a word compares to a child's understanding is another matter.
When a toddler learns a word, such as "pencil," the child will associate the word with the concept of a writing instrument in a variety of ways (even making the mistake of calling a pen a "pencil" after seeing someone write with it). On the other hand, dogs probably learn the word "pen" as a sound that commands a response -- "bring me the pen and you'll get a treat," for example.
Because dogs most likely don't understand abstract concepts, they can't understand the words that refer to abstract concepts. For example, humans understand ideas like "hatred," "beliefs" and "carelessness." These ideas don't necessarily relate to a specific object or action. Ideas that do refer to specific things are called concrete concepts. So, when we tell dogs we love them, this probably doesn't mean as much to them as the word "treat" does. Some might say that until we find a way to interpret a dog's mind, we can't definitively say that dogs don't understand abstract concepts. As far we know right now, however, dogs are only capable of understanding words that refer to concrete things.
Can we say that a dog understands language? That depends on the definition of language, which is contested. If language indicates the process of merely communicating a particular stimulus (a word) to produce a particular reaction, then dogs definitely understand language. But for many linguists, people who study language, a proper definition of language must go deeper.
Some linguists think language necessitates sentences with syntax. Syntax refers to the way words relate to each other in a sentence, based on a structured rule system, such as word order. For instance, for English speakers, the sentence "the dog bites the " means something different from "the man bites the dog," even though both sentences incorporate the same words. Under the umbrella of this stricter definition of language, dogs don't understand language because there's reason to believe they can't understand sentences in this way. Even toddlers can distinguish between parts of speech such as verbs and nouns, while a dog probably can't [source: Kaminski]. One might argue that if dogs can't use syntax like children can, then dogs can't truly understand a word because they don't understand how it relates to other words.
But if dogs can't really comprehend language in the way humans do, why do they seem to understand us so deeply? Certain studies show that dogs pick up on human gestures and cues (like Clever Hans did) better than other animals, even great apes [source: Hare]. So when dogs seem to understand our words, they might really just be reading our body language or tone of voice.
Regardless, dogs have an amazing ability to understand us. For more information on man's best friend, take a look at the links on the next page.