It's simpler than most languages that have a generative grammar.

If you really want to understand it I'd suggest reading this

https://groups.csail.mit.edu/mac/ftpdir/scheme-7.4/doc-html/...

and reading it again, thinking about it, writing some code, reading it again and such. It is astonishing short for a specification for a real programming language and will become clear to you if you live with it. I count 30 "special forms" in there

https://groups.csail.mit.edu/mac/ftpdir/scheme-7.4/doc-html/...

and 24 in the Common Lisp spec

https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node59.html

although this claims the minimum is 9

https://stackoverflow.com/questions/3482389/how-many-primiti...

See also

http://www.paulgraham.com/rootsoflisp.html