In plain words
Of creations I am the beginning, the end, and also the middle, Arjuna. Among branches of knowledge I am Adhyātma-vidyā; among debaters I am reasoned dialogue.
Word for word
Of creations the beginning and the end and the middle too [am] I, O Arjuna; among sciences, adhyātma-vidyā; among those who debate, vāda [am] I.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
सर्गाणां सृष्टीनाम् आदिः अन्तश्च मध्यं चैव अहम् उत्पत्तिस्थितिलयाः अहम् अर्जुन । भूतानां जीवाधिष्ठितानामेव आदिः अन्तश्च इत्याद्युक्तम् उपक्रमे, इह तु सर्वस्यैव सर्गमात्रस्य इति विशेषः । अध्यात्मविद्या विद्यानां मोक्षार्थत्वात् प्रधानमस्मि । वादः अर्थनिर्णयहेतुत्वात् प्रवदतां प्रधानम् , अतः सः अहम् अस्मि । प्रवत्त्कृद्वारेण वदनभेदानामेव वादजल्पवितण्डानाम् इह ग्रहणं प्रवदताम् इति ॥ ३२ ॥
In plain words
Among all creations, all emanations, I am the beginning, the end, and the middle alike — I am their arising, their persisting, and their passing away, Arjuna. Earlier, 'beginning and end' was said only of those creations governed by an individual soul; but here it means something broader — it concerns the whole of creation without exception. Among every branch of knowledge, I am the foremost, because the science of the Self alone has liberation as its very purpose. And among those who engage in speech, I am reasoned debate itself, because it alone settles what is actually true — that is why I am that. What is meant here is specifically the different kinds of speech: debate that seeks truth, debate that seeks victory, and argument that is merely destructive.
Close to the Sanskrit
Of creations, of emanations, I am the beginning and the end and the middle alike — I am origination, persistence, and dissolution, O Arjuna. 'Of beings' — of only those presided over by an individual soul — 'the beginning and the end' was stated at the outset; here, however, the distinction is that it concerns all of creation as such, without restriction. I am foremost among sciences, because the science of the Self has liberation as its very purpose. Reasoned debate, because it is the cause of determining truth, is foremost among speakers; therefore, that I am. By way of this initiating point, what is meant here, among speakers, is specifically the varieties of speech, namely truth-seeking debate, debate for victory, and destructive argument.