In plain words
That buddhi which incorrectly understands dharma and adharma, and what ought and ought not to be done, is rājasī, Partha.
Word for word
By which one knows dharma and adharma, and what is to be done and also what is not to be done, not as they are, that buddhi, O son of Pṛthā, is rājasī.
यया धर्ममधर्मं च कार्यं चाकार्यमेव चअयथावत् प्रजानाति बुद्धिः सा पार्थ राजसी
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
यया धर्मं शास्त्रचोदितम् अधर्मं च तत्प्रतिषिद्धं कार्यं च अकार्यमेव च पूर्वोक्ते एव कार्याकार्ये अयथावत् न यथावत् सर्वतः निर्णयेन न प्रजानाति, बुद्धिः सा पार्थ, राजसी ॥ ३१ ॥
In plain words
The intellect by which you fail to make out dharma — what scripture enjoins — and adharma — what that same scripture forbids — and what must be done and what must not be done, the very duty and non-duty already spoken of: 'ayathāvat' means not as they actually are, never known with a decision settled on every side. That intellect, Pārtha, is rājasic.
Close to the Sanskrit
That by which one does not know dharma — enjoined by scripture — and adharma — forbidden by that same scripture — and what is to be done and what is not to be done, the very duty and non-duty stated earlier, 'ayathāvat', that is, not as it is, does not know it with a determination complete on every side: that intellect, Pārtha, is rājasic.