Children with articulation disorders can be known to do one or more of the following...
delete sounds (child says: /bu/ instead of /bus/)
substitute sounds (child says: /sree/ instead of /three/)
add sounds (child says: /spagbetti/ instead of /spaghetti/
distort sounds (child says: /thpoon/ instead of /spoon/
Individual sounds develop by certain ages. If a sound isn't developed by the expected age, the child could have an articulation disorder.
excerpt from Home speech home website
Typical Speech Sound Development
Sounds in isolation
Sounds in words (i.e., all word positions: initial, medial and final)
Sounds in phrases
Sounds in sentences
Sounds in reading/writing
Sounds in spontaneous conversational speech
Articulation therapy involves significant repetition and targeting of the sounds in error. Although drills are often targeted, a speech pathologist will use a variety of methods to target sounds and keep your child motivated. Once the sound has been mastered at all levels of the hierarchy, it is likely to be remedied with minimal maintenance needed in order to support the best production.
excerpt from North Shore pediatric therapy website