Synonyms:

prakrteh–of material nature; kriyamanani–all being done; gunaih–by the modes; karmani–activities; sarvasah–all kinds of; ahankara-vimudha–bewildered by false ego; atma–the spirit soul; karta–doer; aham–I; iti–thus; manyate–thinks.

Translation:

The bewildered spirit soul, under the influence of the three modes of material nature, thinks himself to be the doer of activities, which are in actuality carried out by nature.