Complex sentence: The initial implementation of Lisp was expected to be a very long process, as compilers at the time were expected to take years to write, and so implementation began with hand-compiling particular functions while the M-expression sublanguage was still tentative.
Split into simpler sentences: The initial implementation of Lisp was expected to be a very long process, as compilers at the time were expected to take years to write. Implementation began with hand-compiling particular functions while the M-expression sublanguage was still tentative.