He (as well as Chomsky) developed the structural linguistic ideas of Leonard Bloomfield
and discovered the linear relations of phonemes and morphemes.
Harris specified two complementary subsets within the grammatical sentences in a particular language (for example, English)
kernel sentences and nonkernel sentences.
The difference between these two subsets is that nonkernel sentences are derived from kernel sentences according to transformational rules.
“The workers rejected the ultimatum” is a kernel sentence from which
nonkernel sentences such as
“The ultimatum was rejected by the workers”
“Did the workers reject the ultimatum?”
may be derived.
hope this helps.