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.
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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.
If, as now seems to be the case, the OP is asking about Old English, then Old English þeod, meaning ‘people’, was a morpheme, and was quite possibly pronounced as /θeɪɒd/, with a syllable boundary after /eɪ/.