Two reading shifts to watch for. (1) As a place-name abbreviation for
台湾 (Taiwan), the syllable is capitalised Tái — same tone, proper-noun convention:
台北,
台商,
港澳台. (2) In a small set of Zhejiang place names,
台 is read tāi (first tone):
天台山 (the mountain),
台州 (the prefecture). Dictionaries still list
天台宗 (the Buddhist school) with second tone tái, but you'll hear locals say tāi for anything geographic in that region.