Original:Water freezes if it were cooled to zero degrees Celsius

Correction: Water freezes if it is cooled to zero degrees Celsius

Doubt:I can make out by reading that is should be used but the official explanation below about subjunctive mood / indicative mood is not clear.

Were (present tense of the hypothetical subjunctive mood) should be is (present tense of the indicative mood). This sentence is stating a general rule that admits of no uncertainty, so the if-clause must be in the indicative mood. You can also omit a tensed verb altogether from the if-clause: Water freezes if cooled to zero degrees Celsius