योगवासिष्ठः yogavāsiṣṭhaḥ - book-1, chapter-16, verse-21

वह्नेरुष्णतरः शैलादपि कष्टतरक्रमः ।
वज्रादपि दृढो ब्रह्मन्दुर्निग्रहमनोग्रहः ॥ २१ ॥

vahneruṣṇataraḥ śailādapi kaṣṭatarakramaḥ ,
vajrādapi dṛḍho brahmandurnigrahamanograhaḥ 21

21. vahneḥ uṣṇataraḥ śailāt api kaṣṭatarakramaḥ
vajrāt api dṛḍhaḥ brahman durnigrahamanograhaḥ

21. O Brahman, this intractable grip of the mind is hotter than fire, its path is more arduous than a mountain, and it is firmer than even a thunderbolt (vajra).

Words meanings summery:

• वह्नेः (vahneḥ) - from fire, than fire

• उष्णतरः (uṣṇataraḥ) - hotter, more burning, more intense

• शैलात् (śailāt) - from a mountain, than a mountain

• अपि (api) - even, also, too

• कष्टतरक्रमः (kaṣṭatarakramaḥ) - having a more difficult course/path, harder to overcome

• वज्रात् (vajrāt) - from a thunderbolt, than a thunderbolt (vajra)

• अपि (api) - even, also, too

• दृढः (dṛḍhaḥ) - firm, strong, hard, solid

• ब्रह्मन् (brahman) - O Brahman, O wise one

• दुर्निग्रहमनोग्रहः (durnigrahamanograhaḥ) - the intractable grip of the mind

Words meanings and morphology

वह्नेः (vahneḥ) - from fire, than fire

(noun)

Ablative, masculine, singular of vahni

vahni - fire, Agni (god of fire)

Note: Used in comparison.

उष्णतरः (uṣṇataraḥ) - hotter, more burning, more intense

(adjective)

Nominative, masculine, singular of uṣṇatara

uṣṇatara - hotter, more burning, more intense

Comparative degree of uṣṇa (hot).

Note: Qualifies 'durnigrahamanograhaḥ'.

शैलात् (śailāt) - from a mountain, than a mountain

(noun)

Ablative, masculine, singular of śaila

śaila - mountain, rock, peak

Note: Used in comparison.

अपि (api) - even, also, too

(indeclinable)

कष्टतरक्रमः (kaṣṭatarakramaḥ) - having a more difficult course/path, harder to overcome

(adjective)

Nominative, masculine, singular of kaṣṭatarakrama

kaṣṭatarakrama - having a more difficult course, harder to traverse or overcome

Compound of kaṣṭatara (more difficult) and krama (course/path).

Compound type : bahuvrīhi (kaṣṭatara+krama)

• kaṣṭatara – more difficult, more arduous
  adjective (masculine)
  Comparative degree of kaṣṭa (difficult).
• krama – course, path, sequence, step
  noun (masculine)
  Root: kram (class 1)

Note: Qualifies 'durnigrahamanograhaḥ'.

वज्रात् (vajrāt) - from a thunderbolt, than a thunderbolt (vajra)

(noun)

Ablative, masculine, singular of vajra

vajra - thunderbolt (Indra's weapon), diamond, hard, firm

Note: Used in comparison.

अपि (api) - even, also, too

(indeclinable)

दृढः (dṛḍhaḥ) - firm, strong, hard, solid

(adjective)

Nominative, masculine, singular of dṛḍha

dṛḍha - firm, strong, hard, solid, fixed

Past Passive Participle (irregularly formed)

Derived from root dṛh.

Root: dṛh (class 1)

Note: Qualifies 'durnigrahamanograhaḥ'.

ब्रह्मन् (brahman) - O Brahman, O wise one

(noun)

Vocative, masculine, singular of brahman

brahman - Brahman (ultimate reality, neuter); priest, wise person (masculine)

दुर्निग्रहमनोग्रहः (durnigrahamanograhaḥ) - the intractable grip of the mind

(noun)

Nominative, masculine, singular of durnigrahamanograha

durnigrahamanograha - the grip of the mind that is difficult to restrain, intractable mental grip

Compound of durnigraha (difficult to restrain), mana (mind), and graha (grip).

Compound type : karmadhāraya-tatpuruṣa (durnigraha+manas+graha)

• durnigraha – difficult to restrain, hard to control, intractable
  adjective (masculine)
  Gerundive/Future Passive Participle (derived from grah with nigrah + dur)
  Derived from root grah with upasargas dur- and ni-.
  Prefixes: dur+ni
  Root: grah (class 9)
• manas – mind, intellect, thought
  noun (neuter)
• graha – seizing, gripping, holding, a seizure, a planet
  noun (masculine)
  Derived from root grah.
  Root: grah (class 9)

Note: This is the subject of the implied verb 'is'.