Lesson

If...not and unless; if and whether

Conditional conjunctions

≈ 20 min 8 block(s)

Text

If...not and unless; if and whether

This lesson explains the difference between 'if...not' and 'unless', and when to use 'if' versus 'whether' in reported yes/no questions. You will learn structures, meanings and typical uses so you can choose the correct form in business and formal writing.

  • 'Unless' generally means 'if not' and introduces an exception.
  • 'If...not' can be used, but often speakers prefer 'unless' for brevity.
  • 'Whether' is used for indirect yes/no questions, especially before infinitives or when both possibilities are considered; 'if' is mainly for conditionals and simple indirect questions.

Table

Comparison: if...not vs unless; if vs whether

Structure Meaning / Use Example
'unless' + clause Means 'if not' — introduces an exception I will attend the meeting unless the client cancels.
'if' + not + clause Can mean the same as 'unless', but is less concise I will attend the meeting if the client does not cancel.
'if' (conditional) Used for conditions and results If the client approves, we will start next week.
'whether' (indirect question) Used to introduce yes/no alternatives or before infinitives We need to decide whether to accept the proposal.
'if' (indirect question) Can introduce a reported yes/no question in informal contexts She asked if the report was ready.

Tip

Key rules — when to use which form

Use these core rules to choose between 'unless', 'if...not', 'if', and 'whether'.

  • Use 'unless' to mean 'if not' when introducing an exception: 'We will proceed unless there is an issue.'
  • Use 'if...not' when you prefer a simple conditional structure: 'If he does not reply, we'll call.'
  • Use 'whether' for indirect yes/no questions, especially before an infinitive or when both options are considered: 'I don't know whether to accept.'
  • Use 'if' for conditional clauses and for informal reported questions: 'She asked if I was available.'

When in doubt: 'unless' = 'if not'; 'whether' is stronger than 'if' when presenting alternatives or before infinitives.

Example

Examples in context

We will sign the contract unless the legal team finds a problem.

If the legal team does not find a problem, we will sign the contract.

The manager asked if the team could meet on Tuesday.

The manager asked whether the team could meet on Tuesday.

Tip

Common mistakes to avoid

Watch out for these universal errors when using these forms.

  • Using 'unless' with a positive meaning (e.g., 'Unless he comes, we will still go' — check logic).
  • Placing 'not' in the wrong clause: 'If not we will cancel' is unclear; prefer 'If we do not receive it, we will cancel.'
  • Replacing 'whether' with 'if' before an infinitive: incorrect — 'decide if to accept' is wrong; use 'decide whether to accept.'
  • Using 'if' in formal writing when 'whether' is required to show alternatives or to be clear.

When in doubt, rewrite the sentence to make the condition or the question explicit.

Quiz

Choose the correct sentence that means: 'I will not attend unless the client approves.'

Hint: Replace 'unless' with 'if not' to check meaning

Quiz

Complete: She asked _____ the meeting would be rescheduled.

Hint: Think about how to introduce a reported yes/no question

Quiz

Choose the best sentence for formal writing:

Hint: Focus on the verb 'decide' followed by an infinitive

Key Points

GrammarPoint

unless

conjunction CEFR B1 //ʌnˈlɛs//

introduces an exception; means 'if not'

We will proceed unless there are objections.

GrammarPoint

if...not

conjunction + negation CEFR B1 //ɪf nɒt//

a conditional structure that expresses 'if not' as an alternative to 'unless'

If the client does not approve, we will delay the launch.

GrammarPoint

whether

conjunction CEFR B2 //ˈwɛðər//

introduces indirect yes/no questions or alternatives; often used before infinitives

We must decide whether to hire an external consultant.

GrammarPoint

if (conditional)

conjunction CEFR B1 //ɪf//

introduces conditional clauses describing possible situations and results

If the team completes the audit, we will publish the report.

Expression

only if

conjunction CEFR B2 //ˈoʊnli ɪf//

indicates a single condition that must be met for something to happen

You will be promoted only if you meet your targets.