1. ‘Scarcely’ and ‘Hardly’ should be followed by when and not by then.

e.g. – Scarcely had I started for the institute, when the rain started.
– Hardly had he arrived, when he had to leave again.

2. ‘No sooner’ is followed by than and not by when.
e.g. – No sooner had I started, than / when the rain started.
– No sooner did he arrive, then / than he had
to leave.

3. ‘Seldom or never’ and ‘seldom if ever’ are both correct, but ‘seldom or ever’ is incorrect.
e.g. – He seldom or never goes to see movies in
theatre.

4.

‘Either-or’, ‘Neither-nor’, ‘not only-but also’, ‘both-and’, ‘whether-or’ etc., should be followed by the same parts of speech or of the same function.
e.g. – He agreed neither to my proposal nor
to his.(✓)

5. The conjunctions ‘though’ (or although) and ‘but’ do the work of setting one statement against another by way of oppositions or contrast and therefore the correlative of ‘though’ is ‘yet’ or a comma (,).
e.g. – Though he worked hard, yet he could not top
the class.

6. In a ‘not only……but also’ sentence, the verb should agree with the noun or pronoun mentioned second, because this is the part being emphasized.
e.g. – Not only the teacher but also the students are enjoying themselves.

7. ‘Such as’ is used to denote a category, whereas ‘such
that’ emphasises the degree of something by
mentioning its consequences.
e.g. – Yuvraj played such an innings as played by the
best batsman.
– Yuvraj played such an innings that it took the
match away from the opposition.

8. ‘Both’ is followed by and not by as well as. Besides,
both has positive sense and cannot be used in negative
sentences.
e.g. – Both Amit  and his friends are coming.
– Neither Amit nor his friends are coming. (✓)

‘Lest’ expresses a negative sense and therefore cannot be used with not. ‘Should’ is always used with ‘lest’.
– Walk steadily, lest you should fall.