There are a number of different kinds of problems that might hinder a person with aphasia from being able to say what they would like to say. The first thing to emphasize is that people with aphasia know what they would like to say, but they have difficulty finding the words they need to communicate their message.
Sometimes a person with aphasia might be able to picture an object, person, place, or other message, and just not be able to think what it is called. This is a phenomenon that happens to all of us occasionally, but for the person with aphasia it can be a constant state. Other times, a person with aphasia might know what they want to say, but a related word comes out instead. For example, the person might say “dog” when they mean to say “cat”. Other times, the sounds that make up the word that they want to say come out in the wrong order. For example, if the person wants to say “table”, it could come out “batle”. This might seem like nonsense sometimes, but it is a problem of the brain failing to select the right sounds for the intended word. Finally, some people with aphasia have great difficulty saying verbs and other small words that are important grammatical words, like articles and prepositions. These individuals sound like they are speaking in telegraphic language. For example, the person might want to say “I went to dinner with my family” but it comes out “Me and family dinner".