Mars is a cold, arid planet, but at one time, it was warm and covered with water. That water is now frozen, but some scientists
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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.
- If + simple present : simple present ( for scientific facts )
Example: If water is boiled , it changes into water vapour.
- If + simple present : Imperative
Example: If you want to win, play fair.
: It expresses possible situation in the present or future.
- If + simple present : will / shall / can / may / must + V¹
Example : I shall play football if I get chance.
: It expresses doubtful situation / hypothetical condition in the present or future
- If + simple past : would / should / could / might + v¹
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.
- If + past perfect : would / should / could / might + have + v³
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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