Insert, append and replace lines or text with SED

A file vik

sed '/keyword/ a\your-text-here' vik #insert your-text-here after every occurance of keyword
sed '5 a\your-text-here' vik #insert your-text-here at line 6
sed '5 i\your-text-here' vik #insert your-text-here at line 5
sed '/^keyword/ i\your-text-here' vik #insert your-text-here before keyword
sed '/^keyword/ c\your-text' vik #replace your keyword with your-text
sed '4 i\text1\ntext2' vik # this will insert two lines