When Correlative Conjunctions are NOT parallel

1. He is as strong as an ox. It is a simile, not a correlative conjunction. Because we can also write it as, 'He is strong as an ox.' The second 'as' is unnecessary, though it would be if it was part of a correlative conjunction.

Other examples: She ran like the wind. (implying speed) His reply was cold as ice. (implying lack of feeling)

In similes, adjectives and verbs are compared to nouns, so this sentence is doesn't violate the rule of parallelism.

2. I was so happy that I could scream. Again, it's not a correlative conjunction. 'So' is an adverb describing 'happy'. How happy? Very happy.

'That I could scream' is an adverbial clause that describes 'happy' as well. How happy was I? I was happy enough to scream.

3. NOT ONLY did he cheat on this exam BUT he ALSO cheated on all exams. First, this sentence is awkward if you omit 'the' before exams. It should read:

'Not only did he cheat on this exam, but he also cheated on all (of) the exams.'

Now, if we are putting to question the inversion, we have to first ask whether the dependent clause can be written in a subject-verb manner. So let's try.

'He not only cheated on this exam, but he also cheated on all the exams.' Here we have an exact parallel between conjoined parts of the sentence. We can omit 'also' to make the sentence flow smoothly, using the conjoining words 'not only' and 'but'.

'He not only cheated on this exam, but he cheated on all the exams.'

4. Twice as many people inhabit China as inhabit India. For convenience's sake, the subject 'people' is omitted in the second part of the sentence. But if we write this sentence in its entirety, we get the following:

'Twice as many people live in China as people live in India.'

By writing out the full sentence, we can see that rule of parallelism is upheld.