In plain words
Among letters I am A; among compounds I am the dvandva. I alone am inexhaustible Time, and I am the dispenser of the fruits of action, facing everywhere.
Word for word
Among letters I am the a, and the dvandva of the group of compounds; I alone [am] inexhaustible time; the dispenser [am] I, facing everywhere.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
अक्षराणां वर्णानाम् अकारः वर्णः अस्मि । द्वन्द्वः समासः अस्मि सामासिकस्य च समाससमूहस्य । किञ्च अहमेव अक्षयः अक्षीणः कालः प्रसिद्धः क्षणाद्याख्यः, अथवा परमेश्वरः कालस्यापि कालः अस्मि । धाता अहं कर्मफलस्य विधाता सर्वजगतः विश्वतोमुखः सर्वतोमुखः ॥ ३३ ॥
In plain words
Among syllables — among the letters, that is — I am the letter 'a'. Among compounds, out of the whole family of them, I am the dvandva. And more: I alone am akṣaya, undiminished — the time you already know, the time called 'moment' and everything measured out from it. Or read it further: as the Supreme Lord, I am the time of time itself. I am dhātā, the one who deals out to the whole world the fruit of what it does; and viśvatomukha means facing every direction at once.
Close to the Sanskrit
Of syllables — that is, of letters — I am the letter 'a'. I am the dvandva compound, of the compound class and of the collection of compounds. Moreover, I alone am akṣaya, undiminished — the well-known time named 'moment' and the rest; or else, as the Supreme Lord, I am the time even of time. I am dhātā, the dispenser of the fruit of action for the whole world; viśvatomukha means facing in all directions.