Rule 1 : The co-relative conjunctions are used in pairs. Not only - but also, Either - or Neither – nor, Both – and, Though – yet, Whether – or. Please see that the pair is properly used.
Ex - They will either do the work or return the money.

Rule 2 : After the adverbs "Hardly / Scarcely", the conjunction 'when or before' should be used.
Ex - Hardly had he left the place when the Bomb Exploded.