Dogs do not understand English, nor they are able to place phrases and sentences into context when they first hear them. However, they can be trained to respond to certain verbal commands in English, such as "sit," a behavior that is termed as "conditioned response."
What some people misjudge as the dog's understanding of English is its response to the tone of human voice. Dogs can be reassured by and respond positively to gentle words. Similarly, sharper tones can be used to get a dog to immediately stop doing a particular action. However, its response to the tone does not indicate understanding of the words being used.