Letter A
In insular script, the letter 'a' is formed by two strokes, creating an 'a' that looks a bit like an 'o' attached to a 'c'.
By the Carolingian period, there were two forms of letter 'a', one majuscule and one minuscule. The majuscule form is like our modern 'A' but without the internal horizontal bar. The minuscule 'a' also looks like our modern 'a'. There is now a rounded stroke above the bowl of the 'a', which continues down and forms the minim.
Letter G
In the insular hand, the 'g' looks like a letter 't' with an additional tail.
However, by the Carolingian period the 'g' has developed a top bowl. The tail also drops down from the right side instead of the left.
Letter S
The insular 's' is much closer to our modern 's'.
The Carolingian 's', however, has lengthened and straightened greatly so that it takes the form of a minuscule 'f' without the crossbar.
Letter T
The 't' in both insular and Carolingian scripts look quite alike. The Carolingian hand is a bit heavier and thicker, but both 't's have a straight top crossbar and a minim with a curved upward stroke at the bottom.