The rules of reduction are actually quite simple, at least on a phonological level. There are four groups of vowels to remember: 1) those that are reduced to /ʊ/ (у, ю), 2) those that are reduced to /ɨ/, 3) those that are reduced to /ɪ/ (е, и, я), and 4) those that are reduced to /ɐ/ or /ə/ (о, а). Next, it is important in which syllable the vowel is found: whether it precedes the stressed syllable (i.e. pre-tonic) or is found elsewhere.

Group 1 (у, ю):
These are always pronounced /ʊ/.

Group 2 (э):
These are always pronounced /ɨ/.

Group 3 (е, и, я):
These are pronounced /ɪ/,
though я is pronounced /ə/ word-finally.

Group 4 (о, а):
These are pronounced /ɐ/ when preceding the stressed syllable.
These are pronounced /ə/ when found elsewhere.


Note that on a phonetic level there are more subtle distinctions among the vowels, but most normal learners do not need to know these rules (or care to learn them). The rules above are sufficient for most, and the subtle distinctions will eventually be picked up by the learner when exposed to spoken Russian.