Firstly, I love this forum,

Secondly, I would like to know whether there are any rules or guidlines to help us know whether a vowel in a word is schwa. i.e pronounced lightly; or do we have to resort to dictionaries.

Thanks in advance.

Best regards ..
In American English, many unstressed vowels become schwas.

e.g. kon sound in conservative (con pronounced with a schwa).

FYI: In American English, the stress tends to be on the second syllable.
If there are any rules I suspect they are long and/or complicated. Rely on the dictionary if you are not sure. The more English you know the more you will be able to "guess" correctly. It is also important to realise that a word may be pronounced differently according to whether the speaker wishes to emphasise the word or is speaking carefully on the one hand or is speaking informally on the other. Short words like to and of always get their full value at the end of a breath group.
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As mentioned above, there are rules and guidelines, but they are complicated.
In general all unstressed syllables are pronounced as schwas.
But that presumes you know which syllables carry the stress, so the whole thing gets a bit circular!

Example: pho - to'' graph - er' The vowels in thesyllables pho and graph are schwas.
pho'' to - graph' The vowel in the syllable to is a schwa.

When the symbol is "e" or "i", the schwa is usually a high schwa (sounds closer to "i" in "bit").
When the symbol is "u" or "o" or "a", the schwa is usually a low schwa (sounds closer to "u" in "but").

There are exceptions to these general rules, and not all speakers conform exactly to the 'rules'.
Unfortunately for the learner. Emotion: sad

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Many thanks to Julielai and Forbes and a BIG thank you to CalifJim
schwa - ə

correction: kən as in conservative. (the schwa can only be viewed in unicode)

Clarify: in American English, the stress tends to be on the second syllabus for a word with 4 syllables or more.