The word alternate can be one of two word classes, with different pronunciations.
- As an adjective, it is pronounced with the stress on the second syllable - 'all-TERN-it', IPA: /ɔːl ˈtɜːrn ət/.
- As a verb 'to alternate'' it is pronounced with the stress on the first syllable, 'ALL-tern-ate', IPA: /ˈɔːl tɜːrn ˌeɪt/.
Properly speaking, the adjective alternate means coming in a repeated changing between two things. An electric motor, or dynamo, has alternate phases of positive and negative (generators like this produce alternating current); the wall of a building may have alternate layers of red and yellow brick; a person may do something on alternate days, for example Mon - Wed - Fri, if alternate days of the week; or, more strictly, Mon - Wed - Fri - Sun - Tues - Thurs - Sat.
In British academic writing, try to avoid the use of the adjective alternate for switching, or choosing, between more than two choices. Many people do use the word where more than one choice is involved; but this is seen by more traditional, and stricter, writers of English as an error.
Similarly, use the verb 'to alternate' for switching between two things, not more. One can alternate between black and white; but not the colours of the rainbow. Electric current which is transmitted in cycles is called alternating current (abbreviated as A.C.).
(As a noun, alternate is not common in British English. In American English, it can be used to mean 'runner-up [in a competition]', 'a reserve [player]', 'a variant [form]' or 'a choice'. Avoid using this noun in British academic writing. It is a trap for the unwary.)`