Complex sentence: The theory of the weak interaction is sometimes called quantum flavourdynamics (QFD), in analogy with the terms QCD and QED, but the term is rarely used because the weak force is best understood in terms of electro-weak theory (EWT).
Split into simpler sentences: The theory of the weak interaction is sometimes called quantum flavourdynamics (QFD), in analogy with the terms QCD dealing with the strong interaction and QED dealing with the electromagnetic force. However the term QFD is rarely used because the weak force is best understood in terms of electro-weak theory (EWT).