Complex sentence: Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container, but under the influence of a magnetic field, it may form structures such as filaments, beams and double layers.
Split into simpler sentences: Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container. Unlike gas, under the influence of a magnetic field, it may form structures such as filaments, beams and double layers.