was/were do do something
'Was/were to do something'
Longman define this phrase as:
Used when talking about a time in the past to say what happened later. Example:
this discovery was to have a major effect on the treatment of heart disease.

I don't thoroughly.comprehend it.
I know the meaning of present tense (is/are to do sth) and the conditional one (if sb was/were to sth), but the simple past form of it, is a kind of ambiguous. Does it mean "unfulfilled intention"?

P.s.
Sorry about the topic sentences, it is a typo.

Question

was/were do do something
'Was/were to do something'
Longman define this phrase as:
Used when talking about a time in the past to say what happened later. Example:
this discovery was to have a major effect on the treatment of heart disease.

I don't thoroughly.comprehend it.
I know the meaning of present tense (is/are to do sth) and the conditional one (if sb was/were to sth), but the simple past form of it, is a kind of ambiguous. Does it mean "unfulfilled intention"?

P.s.
Sorry about the topic sentences, it is a typo.

Answer

It's probably simpler to direct you to this explanation:

http://www.pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/articles/00003.htm

A key phrase is, "They also say that be to + the base form of the verb, when referring to the future, also conveys the connotations of 'requirement' and 'destiny,' ..."

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