Without using 'did' and 'does' in the following sentences we are getting the same meaning then why should we use did / does there.
1.Not only does the number of migrations vary with the month, but also with the type of species.
2.not only did he help her also he dropped her home safely.
Best Answer
You definitely need "did / does" in these sentences.
You want only to be an adverb describing the following verb (did / does) rather than an adjective describing the following noun (the number of migrations / he).
If you say "Not only the number of migrations vary with month ...", it means that number of migrations is not the only thing that is varying with month. This is not what you want the sentence to say and it becomes ungrammatical nonsense with the last part of the sentence.
In the second example you need did to put it in the past tense. Even if you wrote "Not only he helped her also he dropped her home safely.", It would mean that he was not the only one who helped her. Again, this is not what mean and it becomes ungrammatical with the last part of the sentence