BhG 3.27

syntax

prakṛteḥ (of the nature) guṇaiḥ (by the guṇas) sarvaśaḥ (completely) karmāṇi (activities) kriyamāṇāni ([while] being done) [santi] (they are).
[kiṁtu] (but) ahaṅkāra-vimūḍhātmā (whose self is bewildered by egotism) [janaḥ] (a person) manyate (he thinks):
ahaṁ (I) kartā (the doer) [asti] iti (I am).

grammar

prakṛteḥ	–	prakṛti 6n.1 f. – of the nature, primary substance, original cause (from: pra-√kṛ – to produce);
kriyamāṇāni	–	kriyamāṇa (√kṛ – to do) pass. PPr 1n.3 n. – [while] these being done;
guṇaiḥ	–	guṇa 3n.3 m. – by qualities, virtues, threads (from: √grah – to take);
karmāṇi	–	karman 1n.3 n. – activities (from: √kṛ – to do);
sarvaśaḥ	–	av. – completely, entirely, altogether, on all sides (from: sarva – all, whole);
ahaṁkāra-vimūḍhātmā	–	ahaṁkāra-vimūḍhātman 1n.1 m.; BV: yasyātmāhaṁkārena vimūḍho ‘sti saḥ – whose self is bewildered by egotism (from: aham – I; √kṛ – to do, kāra – a doer; ahaṁ-kāra – ego, egotism, pride; vi-√muh – to become confused, bewildered, stupefied, PP vimūḍha –bewildered; ātman – self);
kartā	–	kartṛ 1n.1 m. – the doer (from: √kṛ – to do);
aham	–	asmat sn. 1n.1 – I;
iti	–	av. – thus (used to close the quotation);
manyate	–	√man (to think) Praes. Ā 1v.1 – he thinks;

textual variants

sarvaśaḥ → bhāgaśaḥ (part by part);
kartāham → karmāham (I am the act);

Śāṃkara

The key:
an excerpt from the commented verse
quotes from the scriptures
starting polemic

In what way is an ignorant man attached to actions?		avidvān ajñaḥ kathaṃ karmasu sajjate? ity āha —
Actions are wrought in all cases by the energies of Nature. He whose mind is deluded by egoism thinks ‚I am the doer.’		prakṛteḥ kriyamāṇāni guṇaiḥ karmāṇi sarvaśaḥ | ahaṃkāra-vimūḍhātmā kartāham iti manyate ||3.27||
Nature (Prakṛti, Pradhana) is the equipoised state of the three guṇas or energies, viz., sattva (goodness), rajas (activity), tamas (darkness). It is by the guṇas or the modifications of Nature, manifesting themselves as the body and the senses, that all our actions, conducive to temporal and spiritual ends, are done.		prakṛteḥ prakṛtiḥ pradhānaṃ sattva-rajas-tamasāṃ guṇānāṃ sāmyāvasthā | tasyāḥ prakṛteḥ guṇaiḥ vikāraiḥ kārya-karaṇa-rūpaiḥ kriyamāṇāni karmāṇi laukikāni śāstrīyāṇi ca sarvaśaḥ sarva-prakārair |
The man whose mind (antaḥ-karaṇa) is variously deluded by ahaṁkara, by egoism identifying the aggregate of the body and the senses with the Self, i.e.,		ahaṃkāra-vimūḍhātmā kārya-karaṇa-saṃghātātma-pratyayo ’haṃkāras tena vividhaṃ nānāvidhaṃ mūḍha ātmā antaḥ-karaṇaṃ yasya so ’yaṃ |
who ascribes to himself all the attributes of the body and the senses and thus thoroughly identifies himself with them – he, by nescience, sees actions in himself: as regards every action, he thinks „I am the doer.”		kārya-karaṇa-dharmā kārya-karaṇābhimānī avidyayā karmāṇi ātmani manyamānaḥ tat-tat-karmaṇām ahaṃ karteti manyate ||3.27||

Rāmānuja

commentary under the verse BhG 3.28

Śrīdhara

nanu viduṣo ‚pi cet karma kartavyaṃ tarhi vidvad-aviduṣoḥ ko viśeṣaḥ ? ity āśaṅkyobhayor viśeṣaṃ darśayati prakṛter iti dvābhyām | prakṛter guṇaiḥ prakṛti-kāryair indriyaiḥ sarva-prakāreṇa kriyamāṇāni karmāṇi | tāny aham eva kartā karomīti manyate | atra hetuḥ – ahaṃkāreṇa indriyādiṣv ātmādhyāsena vimūḍha-buddhiḥ san

Madhusūdana

vidvad-aviduṣoḥ karmānuṣṣṭhāna-sāmye ‚pi kartṛtvābhimāna-tad-abhāvābhyāṃ viśeṣaṃ darśayan saktāḥ karmaṇīti-ślokārthaṃ vivṛṇoti dvābhyāṃ prakṛter iti | prakṛtir māyā sattva-rajas-tamo-guṇa-mayī mithyā-jñānātmikā pārameśvarī śaktiḥ māyāṃ tu prakṛtiṃ vidyān māyinaṃ tu maheśvaraṃ iti śruteḥ | tasyāḥ prakṛter guṇair vikāraiḥ kārya-kāraṇa-rūpaiḥ kriyamāṇāni laukikāni vaidikāni ca karmāṇi sarvaśaḥ sarva-prakārair ahaṅkāreṇa kārya-kāraṇa-saṃghātātma-pratyayena vimūḍhaḥ svarūpa-vivekāsamartha ātmāntaḥkaraṇaṃ yasya so ‚haṅkāra-vimūḍhātmānātmany ātmābhimānī tāni karmāṇi kartāham iti karomy aham iti manyate kartṛtvādhyāsena | kartāham iti tṛn-pratyayaḥ | tena na lokāvyaya-niṣṭhā-khal-artha-tṛṇām [Pāṇ 2.3.69] iti ṣaṣṭhī-pratiṣedhaḥ

Viśvanātha

nanu yadi vidvān api karma kuryāt, tarhi vidvad-aviduṣoḥ ko viśeṣaḥ ? ity āśaṅkya tayor viśeṣaṃ darśayati prakṛter iti dvābhyām | prakṛter guṇaiḥ kāryair indriyaiḥ sarvaśaḥ sarva-prakāreṇa kriyamāṇāni yāni karmāṇi tāny aham eva kartā karomīty avidvān manyate

Baladeva

karmitva-sāmye ‚pi vijñājñayor viśeṣam āha prakṛter iti dvābhyām | ahaṃkāra-vimūḍhātmā jano ‚haṃ karmāṇi karteti manyate | na lokāvyaya-niṣṭhā iti sūtrāt ṣaṣṭhī-niṣedhaḥ | karmāṇi laukikāni vaidikāni ca | tāni kīdṛśānīty āha prakṛter īśa-māyāyā guṇais tat-kāryair śarīrendriya-prāṇair īśvara-pravartitaiḥ kriyamāṇānīti | idam eva veditavyam – upakrama-vinirṇayāt saṃvid-vapur-jīvātmāsmad-arthaḥ kartā cānādi-kāla-viṣaya-bhoga-vāsanākrāntas tad-bhogārthikāṃ sva-sannihitāṃ prakṛtim āśliṣṭas tat-kāryeṇāhaṅkāreṇa vimūḍhātmā tādṛśa-sva-vijñāna-śūnyaḥ śarīrādy-ahaṃ-bhāvavān prākṛtaiḥ śarīrādibhir īśena ca siddhāni karmāṇi mayaivaikena kṛtānīti manyate | kartur ātmano yat kartṛtvaṃ tat kila dehādibhis tribhiḥ paramātmanā ca sarva-pravartakena ca siddhyati | na tv ekena jīvenaiva | tac ca mayaiva siddhyatīti jīvo yan manyate tad ahaṅkāra-vimauḍhyād eva – adhiṣṭhānaṃ tathā karthā [Gītā 18.14] ity ādikāc caramādhyāya-vākya-trayāt | kārya-kāraṇa-kartṛtve hetuḥ prakṛtir ucyate [Gītā 13.18] ity atra śarīrendriyādi-kartṛtvaṃ prakṛter iti yad varṇayiṣyate, tatrāpi kevalāyās tasyās tan na śakyaṃ mantum | puruṣa-saṃsargeṇaiva tat-pravṛtter aṅgīkārāt | tataś ca puruṣasya kartṛtvam avarjanīyam iti vyākhyāsyate