but i dont know the mnemonics for 8051
They are in the "bible".
what would be more efficient to work with assebly or C ?,
C is more efficient related to coding time, assembler is more efficient related to execution speed and memory consumption. Neither C nor assembler is efficient if used without a through understading of the '51 architecture. Again read the "bible"
but i still had to learn 8051 assembly
learning assembly is extremely valuable, it will save you spending extreme time debugging your code. Often a "crawl" in the code can not be understood looking at the C code, but looking at the generated assembler will make the reason clear. Then a simple change of the C code will remove the "crawl". One C compiler I once used spent 100 or so more instructions on a for than a while (or was it the other way around, long time ago) and that could never have been found looking at the C code.

Erik