Well, there are lots of reasons why children might have difficulty learning to talk. Sometimes it's merely a delay in the development of speech, and the child will proceed to learn how to talk all on their own.
Other times, there may be particular problems that will require assessment and treatment, perhaps from a speech language pathologist. We usually think about those kinds of problems in two major areas.
The first area is language. If a child has a problem with language development, they are having a problem developing vocabulary, learning what different words are and what they mean. They may have problems understanding what people say to them. They may have difficulty learning particular grammatic forms, like forms of past tense. Or they might have difficulty learning how to put words together in the right order.
The other category, though, that we think about when we're evaluating children is speech. Speech is just the acoustic or the sound of language. When people are talking, that's one form of language. Speech involves movement of the oral articulators — the tongue, the jaw, the lips — to create the sounds of language. So speech is kind of the verbal component of language. And when we evaluate children, we really look at both. Children may have difficulty in both language and speech or have more difficulty in one than the other.
Well, there are many different kinds of speech problems. We think about them in categories. That is, some children have difficulty learning speech, because they have trouble learning a rule-governed system of sounds for their language. So those children might not learn all the sounds or might substitute one sound for another.
But another major category of speech problems are motor speech disorders. And these can happen for different reasons. Sometimes the muscles themselves are weak, and so the sounds are distorted or slurred. Sometimes, however, the muscles might be just fine. But the processes of the brain that tell the muscles what to do aren't working efficiently. And so that's a different kind of a motor speech disorder. And for that kind of motor speech disorder, we use the term childhood apraxia of speech.
Some speech problems are due actually to hearing loss, which might cause the child to produce the sounds incorrectly or not learn the system of sounds for their language. Other children might have structural problems, such as a cleft palate, where the sounds might be distorted because of actual changes in the mouth, particularly the palate. Sometimes even dentition and the way the teeth come together in the mouth can cause some speech problems.