A form is comprised of patterns of movement known as 'postures'.
The term 'posture' is actually very misleading because it suggests a static condition.
It is a mis-translation.
In the 1960's when tai chi chuan was emerging in the West, yoga was very popular and tai chi chuan was mistakenly perceived as being "moving yoga".
Yoga features postures, and tai chi chuan was seen to be similar.
Many tai chi chuan practitioners still think of the art as being akin to yoga.
They are concerned about the end position rather than the entirety of the movement itself.
This constitutes a major error in perception and practice.