In plain words
Darkness, inertia, negligence, and delusion arise when tamas predominates, joy of the Kurus.
Word for word
Lack of light and inactivity, negligence and delusion indeed: these arise when tamas has grown, O joy of the Kurus.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
अप्रकाशः अविवेकः, अत्यन्तम् अप्रवृत्तिश्च प्रवृत्त्यभावः तत्कार्यं प्रमादो मोह एव च अविवेकः मूढता इत्यर्थः । तमसि गुणे विवृद्धे एतानि लिङ्गानि जायन्ते हे कुरुनन्दन ॥ १३ ॥
In plain words
Aprakāśa: no light — meaning no discernment. Atyantam apravṛtti: activity gone completely, a dead stop. Pramāda, heedlessness, is what that dead stop produces. And moha is that same failure to discriminate — plain stupor. That is the sense. When tamas has swelled to full strength, these are the signs that appear, son of the Kurus.
Close to the Sanskrit
Aprakāśa — non-discrimination; and atyantam apravṛtti — the absence of activity; its effect, pramāda; and moha too — non-discrimination, dullness: this is the meaning. When the guṇa tamas has grown full, these signs arise, O Kurunandana.