I met a word τοὐναντίον, and my textbook said that it is τὸ+ἐναντίον. I wonder that is this common? And why?
It’s called ‘crasis’, and can happen when a word ending in a vowel is followed by a word beginning with a vowel (‘hiatus’). The two words are combined, and contract in the usual way.
In this case, the ο and ε form the long vowel ου, just as in the εω contract verbs (e.g. φιλέομεν = φιλοῦμεν).
It is very common, and occurs to avoid hiatus (we also avoid it in English when we says things like ‘an apple’ instead of 'a apple).