unable

unable — adjective

1. when someone is unable to do something, they cannot achieve it because they lack

1.adjectiveA2
Definition

when someone is unable to do something, they cannot achieve it because they lack the necessary ability, time, or opportunity

Example

Minh was unable to attend the meeting because his flight was delayed.

be unable + to-infinitive (reason clause)

The hospital was unable to admit more patients after the power failure.

institutional subject with be unable

Common collocations
  • unable to afford
  • unable to cope
  • unable to move
  • unable to speak
  • unable to sleep
Synonyms
  • can't

    informal equivalent; used in everyday speech instead of 'unable'

  • incapable

    stronger than 'unable'; suggests a permanent or fundamental lack of ability rather than a temporary one

  • powerless

    focuses on lack of authority or control rather than lack of skill

Antonyms
  • able

    direct opposite; can be used before a noun (an able teacher) or with to-infinitive (able to help)

  • capable

    similar to 'able'; usually followed by 'of' (capable of doing)

Grammar Patterns

be unable + to-infinitive

feel unable + to-infinitive

remain unable + to-infinitive

Usage Note

Unlike most adjectives, 'unable' cannot be placed before a noun (❌ an unable person). It must always be followed by a to-infinitive clause. In casual conversation, 'can't' is far more common than 'unable'; use 'unable' mainly in formal writing or when you want to sound polite and serious.

Common Mistakes

I am unable going to the party.
I am unable to go to the party.
'unable' must be followed by 'to + infinitive', not a gerund.
He is an unable student.
He is unable to pass the exam.
'unable' cannot go before a noun; use 'not a capable' or restructure the sentence.

Word family

Synonym discussion

Sense: adjective/1

'Unable' is the most neutral and formal of the four — it simply states that something cannot be done, without saying why. 'Can't' is its everyday, informal counterpart and is far more common in speech. 'Incapable' is stronger and often suggests a lasting limitation in character or ability (e.g., 'He is incapable of telling a lie'). 'Powerless' shifts the focus from skill to control — it means you want to act but lack the authority or means (e.g., 'The police were powerless to stop the crowd'). Use 'unable' in essays, reports, and polite refusals; use 'can't' in conversation; reserve 'incapable' and 'powerless' for situations where the reason matters.

unable ↗can'tincapablepowerless ↗

Common collocations

Adverbs

  • quite unable
  • completely unable
  • physically unable
  • simply unable

Phrases

  • unable to cope
  • unable to afford
  • unable to speak

Etymology

LatinOld FrenchEnglish

Formed in late Middle English by adding the prefix 'un-' (meaning 'not') to 'able'. The word 'able' itself comes from Latin 'habilis' ('easy to handle, fit') via Old French 'able'. The modern sense of 'lacking the means or capacity to do something' has been in use since the 14th century.