Strong and Weak forms of pronunciation is a fairly advanced topic. It is taught to ESL students (at least in the UK) to intermediate/higher learners.
As you mentioned, one of the best ways to pronounce wods is to learn the phonetic alphabet (the inverted 'e' you refer to, called a schwa, is part of that alphabet).
Even I didn't realise quite how many pronunciations of to there were!:
Definition of to preposition (INFINITIVE) from Cambridge Dictionary Online: Free English Dictionary and Thesaurus
It's also useful to listen to spoken English whilst following it with a transcript, noticing strong and weak forms of words.