What is conditional sentences

ExplanationB

Sentences containing conditional clauses like 'if' , 'unless and 'provide that' are called conditional sentences. There are four types and express four types of meaning.

It always expresses real and possible.

Example: If water is boiled , it changes into water vapour.

Example: If you want to win, play fair.

: It expresses possible situation in the present or future.

Example : I shall play football if I get chance.

: It expresses doubtful situation / hypothetical condition in the present or future

Example : If I were you , I would never accept the proposal.

Note: ' Be' verb in if clause always becomes ' were ' ( only in conditional sentence type two)

: It expresses regret / unfulfilled situation in the past.

Example : He would have helped you if you had asked him.

' Unless ' has the similar structure like ' if ' but it gives negative / situational meaning.

Example : She wouldn't learn much unless she worked harder.

' Provided that ' functions like ' if ".

Example: Provided that you were a teacher , what would you do?

The auxiliary verb of conditional sentence type two and three can be fronted in place of ' if'

Example: Were I a bird , I would fly.

Hope I helped!

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c, his tragic flaw to cause his downfall

your answer is c) experiments with water vapor showed that elements combine in specific proportions.