Can vowels be combined in English without forming diphthongs?

Usually all combinations of vowels in English function as diphthongs. Are there any combinations of vowels in English that do not function as diphthongs? if there are no such examples - I would be grateful to you if you mention the source, where this restriction is mentioned as a combinatory rule for English vowel phonemes.

You are absolutely right -my question didn't sound precise. I meant the phonotactics of Old English - were there any cases of vowel combinations within one morpheme that were not diphthongs? Something like CV+VC or CCV+V

(A later comment from the original poster)

Examples within a single morpheme exist (e.g., pIAno, & nAIve), and across morpheme boundaries, it would be very common (e.g., gOIng). These are not diphthongs because the two vowels occur in different syllables.